Cardiac electrical signal gross morphology-based noise detection for rejection of ventricular tachyarrhythmia detection

ABSTRACT

A medical device system, such as an extra-cardiovascular implantable cardioverter defibrillator ICD, senses R-waves from a first cardiac electrical signal by a first sensing channel and stores a time segment of a second cardiac electrical signal in response to each sensed R-wave. The medical device system determines a morphology parameter correlated to signal noise from time segments of the second cardiac electrical signal, detects a noisy signal segment based on the signal morphology parameter; and withholds detection of a tachyarrhythmia episode in response to detecting a threshold number of noisy signal segments.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Patent Application No.62/367,166, filed provisionally on Jul. 27, 2016, the entire content ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates generally to a medical device system and methodfor detecting noise in a cardiac electrical signal and withholding aventricular tachyarrhythmia detection in response to detecting noise.

BACKGROUND

Medical devices, such as cardiac pacemakers and implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to aheart of a patient via electrodes carried by one or more medicalelectrical leads and/or electrodes on a housing of the medical device.The electrical stimulation may include signals such as pacing pulses orcardioversion or defibrillation shocks. In some cases, a medical devicemay sense cardiac electrical signals attendant to the intrinsic orpacing-evoked depolarizations of the heart and control delivery ofstimulation signals to the heart based on sensed cardiac electricalsignals. Upon detection of an abnormal rhythm, such as bradycardia,tachycardia or fibrillation, an appropriate electrical stimulationsignal or signals may be delivered to restore or maintain a more normalrhythm of the heart. For example, an ICD may deliver pacing pulses tothe heart of the patient upon detecting bradycardia or tachycardia ordeliver cardioversion or defibrillation shocks to the heart upondetecting tachycardia or fibrillation. The ICD may sense the cardiacelectrical signals in a heart chamber and deliver electrical stimulationtherapies to the heart chamber using electrodes carried by transvenousmedical electrical leads. Cardiac signals sensed within the heartgenerally have a high signal strength and quality for reliably sensingcardiac electrical events, such as R-waves. In other examples, anon-transvenous lead may be coupled to the ICD, in which case cardiacsignal sensing presents new challenges in accurately sensing cardiacelectrical events.

SUMMARY

In general, the disclosure is directed to techniques for detectingnoise-contaminated cardiac electrical signal segments and reject aventricular tachyarrhythmia detection in response to detecting noisysignal segments. A medical device system operating according to thetechniques disclosed herein may determine signal morphology parametersfor a cardiac electrical signal segment that extends beyond an expectedQRS signal width for identifying noisy signal segments based on themorphology parameters.

In one example, the disclosure provides an extra-cardiovascular ICDincluding a sensing circuit having a first sensing channel configured toreceive a first cardiac electrical signal and sense R-waves in responseto crossings of a sensing amplitude threshold by the first cardiacelectrical signal and having a second sensing channel configured toreceive a second cardiac electrical signal. The second sensing channelreceives the second cardiac electrical signal via anextra-cardiovascular sensing electrode vector and different than anextra-cardiovascular sensing electrode vector used by the first sensingchannel. The ICD further includes a memory and a control circuit coupledto the sensing circuit and the memory. The control circuit is configuredto store a time segment of the second cardiac electrical signal in thememory for each of the plurality of R-waves sensed by the first sensingchannel. The control circuit is configured to, for each of a pluralityof the stored time segments, determine a morphology parameter correlatedto signal noise from the stored time segment and detect the stored timesegment as being a noisy signal segment based on the morphologyparameter determined for the respective stored time segment. The controlcircuit withholds detection of a tachyarrhythmia episode in response todetecting at least a threshold number of the stored time segments asnoisy signal segments.

In another example, the disclosure provides a method performed by anextra-cardiovascular ICD including sensing R-waves by a first sensingchannel of a sensing circuit of the extra-cardiovascular ICD in responseto crossings of a sensing amplitude threshold by a first cardiacelectrical signal, the first cardiac electrical signal received by thefirst sensing channel via a first extra-cardiovascular sensing electrodevector coupled to the extra-cardiovascular ICD; storing a time segmentof a second cardiac electrical signal in a memory of the for each of theplurality of R-waves sensed by the first sensing channel, the secondcardiac electrical signal received via a second extra-cardiovascularsensing electrode vector coupled to the extra-cardiovascular ICD anddifferent than the first extra-cardiovascular sensing electrode vector.The method further includes, for each of a plurality of the stored timesegments, determining a morphology parameter correlated to signal noisefrom the stored time segment; detecting the stored time segment as beinga noisy signal segment based on the morphology parameter determined forthe respective stored time segment; and withholding detection of atachyarrhythmia episode in response to detecting at least a thresholdnumber of the stored time segments as noisy signal segments

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of an extra-cardiovascular ICD, causethe extra-cardiovascular ICD to sense R-waves by a first sensing channelof a sensing circuit of the extra-cardiovascular ICD in response tocrossings of a sensing amplitude threshold by a first cardiac electricalsignal, the first cardiac electrical signal received by the firstsensing channel via a first extra-cardiovascular sensing electrodevector coupled to the extra-cardiovascular ICD; store a time segment ofa second cardiac electrical signal in ICD memory in response to each oneof the plurality of R-waves sensed by the first sensing channel, thesecond cardiac electrical signal received via a secondextra-cardiovascular sensing electrode vector by a second sensingchannel of the extra-cardiovascular ICD; for each one of a plurality ofthe stored time segments determine a morphology parameter correlated tosignal noise from the stored time segment and detect the stored timesegment as being a noisy signal segment based on the morphologyparameter determined for the respective stored time segment; andwithhold detection of a tachyarrhythmia episode in response to detectingat least a threshold number of the stored time segments as noisy signalsegments.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of a patient implanted with theextra-cardiovascular ICD system of FIG. 1A in a different implantconfiguration.

FIG. 3 is a conceptual diagram of a distal portion of anextra-cardiovascular lead having an electrode configuration according toanother example.

FIG. 4 is a schematic diagram of the ICD of FIGS. 1A-2C according to oneexample.

FIG. 5 is diagram of circuitry included in the sensing circuit of FIG. 4according to one example.

FIG. 6 is a plot of the attenuation characteristics of a notch filterthat may be included in the sensing circuit of FIG. 5.

FIG. 7 is a flow chart of a method performed by the ICD of FIGS. 1A-2Cfor sensing and confirming R-waves for use in tachyarrhythmia detectionaccording to one example.

FIG. 8 is a diagram of a filtered cardiac electrical signal and anamplitude ratio threshold that may be applied for confirming R-wavesensed events.

FIG. 9 is an example of a look-up table of amplitude ratio thresholdvalues that may be stored in memory of the ICD of FIGS. 1A-2C for use inconfirming R-wave sensed events.

FIG. 10 is a flow chart of a method for detecting tachyarrhythmia by anICD according to one example.

FIG. 11 is a flow chart of a method for detecting tachyarrhythmia by anICD according to another example.

FIG. 12 is a diagram of circuitry included in the sensing circuit ofFIG. 4 according to another example.

FIG. 13 is a flow chart of a method for detecting tachyarrhythmia by anICD according to yet another example.

FIG. 14 is a flow chart of a method performed by an ICD for withholdinga ventricular tachyarrhythmia detection in response to a grossmorphology rejection rule.

FIGS. 15, 16 and 17 are flow charts of illustrative methods fordetermining gross morphology parameters of a cardiac electrical signalthat may be used in detecting a noisy signal segment.

FIG. 18 is a flow chart of a method for comparing the gross morphologyparameters of FIGS. 15, 16 and 17 to noisy segment detection criteriaand detecting a cardiac electrical signal segment as a noisy segment.

FIG. 19 is a timing diagram depicting time segments over which amorphology parameter correlated to baseline noise, an amplitudeparameter and a signal width parameter may be determined for determiningwhether the respective time segment is a noisy segment.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for detecting noisecontamination of a cardiac electrical signal in a medical device systemand withholding detection of a ventricular tachyarrhythmia in responseto detecting noise. The medical device system may be any implantable orexternal medical device enabled for sensing cardiac electrical signals,including implantable pacemakers, implantablecardioverter-defibrillators (ICDs), cardiac resynchronization therapy(CRT) devices, or cardiac monitors coupled to extra-cardiovascular,transvenous, epicardial or intrapericardial leads carrying sensingelectrodes; leadless pacemakers, ICDs or cardiac monitors havinghousing-based sensing electrodes; and external pacemakers,defibrillators, or cardiac monitors coupled to external, surface or skinelectrodes.

However, the techniques are described in conjunction with an implantablemedical lead carrying extra-cardiovascular electrodes. As used herein,the term “extra-cardiovascular” refers to a position outside the bloodvessels, heart, and pericardium surrounding the heart of a patient.Implantable electrodes carried by extra-cardiovascular leads may bepositioned extra-thoracically (outside the ribcage and sternum) orintra-thoracically (beneath the ribcage or sternum) but generally not inintimate contact with myocardial tissue. The techniques disclosed hereinprovide a method for detecting noisy signals acquired usingextra-cardiovascular electrodes and withhold detection of ventriculartachycardia (VT) and ventricular fibrillation (VF) when signal noise isidentified.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIGS. 1A and 1B are described in the context of an ICD system10 capable of providing defibrillation and/or cardioversion shocks andpacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as an electrode (sometimes referred to as a canelectrode). Housing 15 may be used as an active can electrode for use indelivering cardioversion/defibrillation (CV/DF) shocks or other highvoltage pulses delivered using a high voltage therapy circuit. In otherexamples, housing 15 may be available for use in delivering unipolar,low voltage cardiac pacing pulses in conjunction with lead-based cathodeelectrodes and for sensing cardiac electrical signals in conjunctionwith lead-based electrodes. In other instances, the housing 15 of ICD 14may include a plurality of electrodes on an outer portion of thehousing. The outer portion(s) of the housing 15 functioning as anelectrode(s) may be coated with a material, such as titanium nitride.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,electrical cardiac signal sensing circuitry, therapy delivery circuitry,power sources and other components for sensing cardiac electricalsignals, detecting a heart rhythm, and controlling and deliveringelectrical stimulation pulses to treat an abnormal heart rhythm.

Lead 16 includes an elongated lead body 18 having a proximal end 27 thatincludes a lead connector (not shown) configured to be connected to ICDconnector assembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead 16 includes defibrillation electrodes 24 and 26 andpace/sense electrodes 28, 30 and 31. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently. In some instances, defibrillation electrodes 24and 26 are coupled to electrically isolated conductors, and ICD 14 mayinclude switching mechanisms to allow electrodes 24 and 26 to beutilized as a single defibrillation electrode (e.g., activatedconcurrently to form a common cathode or anode) or as separatedefibrillation electrodes, (e.g., activated individually, one as acathode and one as an anode or activated one at a time, one as an anodeor cathode and the other remaining inactive with housing 15 as an activeelectrode).

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to low voltage pacing and sensing electrodes 28, 30 and31. However, electrodes 24 and 26 and housing 15 may also be utilized toprovide pacing functionality, sensing functionality or both pacing andsensing functionality in addition to or instead of high voltagestimulation therapy. In this sense, the use of the term “defibrillationelectrode” herein should not be considered as limiting the electrodes 24and 26 for use in only high voltage cardioversion/defibrillation shocktherapy applications. For example, electrodes 24 and 26 may be used in apacing electrode vector for delivering extra-cardiovascular pacingpulses such as ATP pulses and/or in a sensing vector used to sensecardiac electrical signals and detect VT and VF.

Electrodes 28, 30 and 31 are relatively smaller surface area electrodesfor delivering low voltage pacing pulses and for sensing cardiacelectrical signals. Electrodes 28, 30 and 31 are referred to aspace/sense electrodes because they are generally configured for use inlow voltage applications, e.g., used as either a cathode or anode fordelivery of pacing pulses and/or sensing of cardiac electrical signals.In some instances, electrodes 28, 30 and 31 may provide only pacingfunctionality, only sensing functionality or both.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. A third pace/senseelectrode 31 may be located distal to defibrillation electrode 26.Electrodes 28 and 30 are illustrated as ring electrodes, and electrode31 is illustrated as a hemispherical tip electrode in the example ofFIGS. 1A and 1B. However, electrodes 28, 30 and 31 may comprise any of anumber of different types of electrodes, including ring electrodes,short coil electrodes, hemispherical electrodes, directional electrodes,segmented electrodes, or the like, and may be positioned at any positionalong the distal portion 25 of lead 16 and are not limited to thepositions shown. Further, electrodes 28, 30 and 31 may be of similartype, shape, size and material or may differ from each other.

Lead 16 extends subcutaneously or submuscularly over the ribcage 32medially from the connector assembly 27 of ICD 14 toward a center of thetorso of patient 12, e.g., toward xiphoid process 20 of patient 12. At alocation near xiphoid process 20, lead 16 bends or turns and extendssuperior subcutaneously or submuscularly over the ribcage and/orsternum, substantially parallel to sternum 22. Although illustrated inFIGS. 1A and 1B as being offset laterally from and extendingsubstantially parallel to sternum 22, lead 16 may be implanted at otherlocations, such as over sternum 22, offset to the right or left ofsternum 22, angled laterally from sternum 22 toward the left or theright, or the like. Alternatively, lead 16 may be placed along othersubcutaneous or submuscular paths. The path of extra-cardiovascular lead16 may depend on the location of ICD 14, the arrangement and position ofelectrodes carried by the lead distal portion 25, and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, 30 and 31 locatedalong the distal portion 25 of the lead body 18. Lead body 18 may betubular or cylindrical in shape. In other examples, the distal portion25 (or all of) the elongated lead body 18 may have a flat, ribbon orpaddle shape. The lead body 18 of lead 16 may be formed from anon-conductive material, including silicone, polyurethane,fluoropolymers, mixtures thereof, and other appropriate materials, andshaped to form one or more lumens within which the one or moreconductors extend. However, the techniques disclosed herein are notlimited to such constructions or to any particular lead body design.

The elongated electrical conductors contained within the lead body 18are each electrically coupled with respective defibrillation electrodes24 and 26 and pace/sense electrodes 28, 30 and 31, which may be separaterespective insulated conductors within the lead body. The respectiveconductors electrically couple the electrodes 24, 26, 28, 30 and 31 tocircuitry, such as a therapy delivery circuit and/or a sensing circuit,of ICD 14 via connections in the connector assembly 17, includingassociated electrical feedthroughs crossing housing 15. The electricalconductors transmit therapy from a therapy delivery circuit within ICD14 to one or more of defibrillation electrodes 24 and 26 and/orpace/sense electrodes 28, 30 and 31 and transmit sensed electricalsignals from one or more of defibrillation electrodes 24 and 26 and/orpace/sense electrodes 28, 30 and 31 to the sensing circuit within ICD14.

ICD 14 may obtain electrical signals corresponding to electricalactivity of heart 8 via a combination of sensing vectors that includecombinations of electrodes 28, 30, and/or 31. In some examples, housing15 of ICD 14 is used in combination with one or more of electrodes 28,30 and/or 31 in a sensing electrode vector. ICD 14 may even obtaincardiac electrical signals using a sensing vector that includes one orboth defibrillation electrodes 24 and/or 26, e.g., between electrodes 24and 26 or one of electrodes 24 or 26 in combination with one or more ofelectrodes 28, 30, 31, and/or the housing 15.

ICD 14 analyzes the cardiac electrical signals received from one or moreof the sensing vectors to monitor for abnormal rhythms, such asbradycardia, VT or VF. ICD 14 may analyze the heart rate and/ormorphology of the cardiac electrical signals to monitor fortachyarrhythmia in accordance with any of a number of tachyarrhythmiadetection techniques. One example technique for detectingtachyarrhythmia is described in U.S. Pat. No. 7,761,150 (Ghanem, etal.), incorporated by reference herein in its entirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF). ICD 14 may deliver ATPin response to VT detection, and in some cases may deliver ATP prior toa CV/DF shock or during high voltage capacitor charging in an attempt toavert the need for delivering a CV/DF shock. ATP may be delivered usingan extra-cardiovascular pacing electrode vector selected from any ofelectrodes 24, 26, 28, 30, 31 and/or housing 15. The pacing electrodevector may be different than the sensing electrode vector. In oneexample, cardiac electrical signals are sensed between pace/senseelectrodes 28 and 30 and between one of pace/sense electrodes 28 or 30and housing 15, and ATP pulses are delivered between pace/senseelectrode 30 used as a cathode electrode and defibrillation electrode 24used as a return anode electrode. In other examples, pacing pulses maybe delivered between pace/sense electrode 28 and either (or both)defibrillation electrode 24 or 26 or between defibrillation electrode 24and defibrillation electrode 26. These examples are not intended to belimiting, and it is recognized that other sensing electrode vectors andpacing electrode vectors may be selected according to individual patientneed.

If ATP does not successfully terminate VT or when VF is detected, ICD 14may deliver one or more cardioversion or defibrillation (CV/DF) shocksvia one or both of defibrillation electrodes 24 and 26 and/or housing15. ICD 14 may deliver the CV/DF shocks using electrodes 24 and 26individually or together as a cathode (or anode) and with the housing 15as an anode (or cathode). ICD 14 may generate and deliver other types ofelectrical stimulation pulses such as post-shock pacing pulses orbradycardia pacing pulses using a pacing electrode vector that includesone or more of the electrodes 24, 26, 28, 30 and 31 and the housing 15of ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. In otherexamples, lead 16 may include less than three pace/sense electrodes ormore than three pace/sense electrodes and/or a single defibrillationelectrode or more than two electrically isolated or electrically coupleddefibrillation electrodes or electrode segments. The pace/senseelectrodes 28, 30 and/or 31 may be located elsewhere along the length oflead 16. For example, lead 16 may include a single pace/sense electrode30 between defibrillation electrodes 24 and 26 and no pace/senseelectrode distal to defibrillation electrode 26 or proximaldefibrillation electrode 24. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the extra-cardiovascular pacingtechniques disclosed herein are described in U.S. Publication No.2015/0306375 (Marshall, et al.) and U.S. Publication No. 2015/0306410(Marshall, et al.), both of which are incorporated herein by referencein their entirety.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferiorly from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.Lead 16 may be implanted in other extra-cardiovascular locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion 25 of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may be used to program R-wave sensingparameters, cardiac rhythm detection parameters and therapy controlparameters used by ICD 14. Data stored or acquired by ICD 14, includingphysiological signals or associated data derived therefrom, results ofdevice diagnostics, and histories of detected rhythm episodes anddelivered therapies, may be retrieved from ICD 14 by external device 40following an interrogation command. External device 40 may alternativelybe embodied as a home monitor or hand held device.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement,extra-cardiovascular lead 16 of system 10 is implanted at leastpartially underneath sternum 22 of patient 12. Lead 16 extendssubcutaneously or submuscularly from ICD 14 toward xiphoid process 20and at a location near xiphoid process 20 bends or turns and extendssuperiorly within anterior mediastinum 36 in a substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22.In some instances, the anterior wall of anterior mediastinum 36 may alsobe formed by the transversus thoracis muscle and one or more costalcartilages. Anterior mediastinum 36 includes a quantity of looseconnective tissue (such as areolar tissue), adipose tissue, some lymphvessels, lymph glands, substernal musculature, small side branches ofthe internal thoracic artery or vein, and the thymus gland. In oneexample, the distal portion 25 of lead 16 extends along the posteriorside of sternum 22 substantially within the loose connective tissueand/or substernal musculature of anterior mediastinum 36.

A lead implanted such that the distal portion 25 is substantially withinanterior mediastinum 36 may be referred to as a “substernal lead.” Inthe example illustrated in FIGS. 2A-2C, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. In some instances, lead 16 may extend laterally such that distalportion 25 of lead 16 is underneath/below the ribcage 32 in addition toor instead of sternum 22. In other examples, the distal portion 25 oflead 16 may be implanted in other extra-cardiovascular, intra-thoraciclocations, including the pleural cavity or around the perimeter of andadjacent to but typically not within the pericardium 38 of heart 8.Other implant locations and lead and electrode arrangements that may beused in conjunction with the cardiac pacing techniques described hereinare generally disclosed in the above-incorporated patent applications.

FIG. 3 is a conceptual diagram illustrating a distal portion 25′ ofanother example of extra-cardiovascular lead 16 of FIGS. 1A-2C having acurving distal portion 25′ of lead body 18′. Lead body 18′ may be formedhaving a curving, bending, serpentine, or zig-zagging shape along distalportion 25′. In the example shown, defibrillation electrodes 24′ and 26′are carried along curving portions of the lead body 18′. Pace/senseelectrode 30′ is carried in between defibrillation electrodes 24′ and26′. Pace/sense electrode 28′ is carried proximal to the proximaldefibrillation electrode 24′. No electrode is provided distal todefibrillation electrode 26′ in this example.

As shown in FIG. 3, lead body 18′ may be formed having a curving distalportion 25′ that includes two “C” shaped curves, which together mayresemble the Greek letter epsilon, “ε.” Defibrillation electrodes 24′and 26′ are each carried by one of the two respective C-shaped portionsof the lead body distal portion 25′, which extend or curve in the samedirection away from a central axis 33 of lead body 18′. In the exampleshown, pace/sense electrode 28′ is proximal to the C-shaped portioncarrying electrode 24′, and pace/sense electrode 30′ is proximal to theC-shaped portion carrying electrode 26′. Pace/sense electrodes 28′ and30′ may, in some instances, be approximately aligned with the centralaxis 33 of the straight, proximal portion of lead body 18′ such thatmid-points of defibrillation electrodes 24′ and 26′ are laterally offsetfrom electrodes 28′ and 30′. Other examples of extra-cardiovascularleads including one or more defibrillation electrodes and one or morepacing and sensing electrodes carried by curving, serpentine, undulatingor zig-zagging distal portion of the lead body that may be implementedwith the pacing techniques described herein are generally disclosed inU.S. Pat. Publication No. 2016/0158567 (Marshall, et al.), incorporatedherein by reference in its entirety.

FIG. 4 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 4) includes software, firmware and hardware thatcooperatively monitor cardiac electrical signals, determine when anelectrical stimulation therapy is necessary, and deliver therapies asneeded according to programmed therapy delivery algorithms and controlparameters. The software, firmware and hardware are configured to detecttachyarrhythmias and deliver anti-tachyarrhythmia therapy, e.g., detectventricular tachyarrhythmias and in some cases discriminate VT and VFfor determining when ATP or CV/DF shocks are required. ICD 14 is coupledto an extra-cardiovascular lead, such as lead 16 carryingextra-cardiovascular electrodes 24, 26, 28, 30 and 31 (if present), fordelivering electrical stimulation pulses to the patient's heart and forsensing cardiac electrical signals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, sensing circuit 86, and telemetry circuit 88. A power source98 provides power to the circuitry of ICD 14, including each of thecomponents 80, 82, 84, 86, and 88 as needed. Power source 98 may includeone or more energy storage devices, such as one or more rechargeable ornon-rechargeable batteries. The connections between power source 98 andeach of the other components 80, 82, 84, 86 and 88 are to be understoodfrom the general block diagram of FIG. 4, but are not shown for the sakeof clarity. For example, power source 98 may be coupled to a low voltage(LV) charging circuit and to a high voltage (HV) charging circuitincluded in therapy delivery circuit 84 for charging low voltage andhigh voltage capacitors, respectively, included in therapy deliverycircuit 84 for producing respective low voltage pacing pulses, such asbradycardia pacing, post-shock pacing or ATP pulses, or for producinghigh voltage pulses, such as CV/DF shock pulses. In some examples, highvoltage capacitors are charged and utilized for delivering ATP,post-shock pacing or other pacing pulses instead of low voltagecapacitors. Power source 98 is also coupled to components of sensingcircuit 86, such as sense amplifiers, analog-to-digital converters,switching circuitry, etc. as needed.

The functional blocks shown in FIG. 4 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsor combinations of components that provide the described functionality.The particular form of software, hardware and/or firmware employed toimplement the functionality disclosed herein will be determinedprimarily by the particular system architecture employed in the ICD andby the particular detection and therapy delivery methodologies employedby the ICD. Providing software, hardware, and/or firmware to accomplishthe described functionality in the context of any modern ICD system,given the disclosure herein, is within the abilities of one of skill inthe art.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80 orother ICD components to perform various functions attributed to ICD 14or those ICD components. The non-transitory computer-readable mediastoring the instructions may include any of the media listed above.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as components isintended to highlight different functional aspects and does notnecessarily imply that such components must be realized by separatehardware or software components. Rather, functionality associated withone or more components may be performed by separate hardware, firmwareor software components, or integrated within common hardware, firmwareor software components. For example, cardiac event sensing andtachyarrhythmia detection operations may be performed by sensing circuit86 under the control of control circuit 80 and may include operationsimplemented in a processor or other signal processing circuitry includedin control circuit 80 executing instructions stored in memory 82 andcontrol signals such as blanking and timing intervals and sensingthreshold amplitude signals sent from control circuit 80 to sensingcircuit 86.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and sensing circuit 86 for sensing cardiacelectrical activity, detecting cardiac rhythms, and controlling deliveryof cardiac electrical stimulation therapies in response to sensedcardiac signals. Therapy delivery circuit 84 and sensing circuit 86 areelectrically coupled to electrodes 24, 26, 28, 30 and 31 (if present asshown in FIGS. 1A and 2A) carried by lead 16 (e.g., as shown in FIGS.1A-3) and the housing 15, which may function as a common or groundelectrode or as an active can electrode for delivering CV/DF shockpulses or cardiac pacing pulses.

Sensing circuit 86 may be selectively coupled to electrodes 28, 30, 31and/or housing 15 in order to monitor electrical activity of thepatient's heart. Sensing circuit 86 may additionally be selectivelycoupled to defibrillation electrodes 24 and/or 26 for use in a sensingelectrode vector. Sensing circuit 86 is enabled to selectively receivecardiac electrical signals from at least two sensing electrode vectorsfrom the available electrodes 24, 26, 28, 30, 31 and housing 15. Atleast two cardiac electrical signals from two different sensingelectrode vectors may be received simultaneously by sensing circuit 86,and sensing circuit 86 may monitor one or both or the cardiac electricalsignals at a time for sensing cardiac electrical signals. For example,sensing circuit 86 may include switching circuitry for selecting whichof electrodes 24, 26, 28, 30, 31 and housing 15 are coupled to a sensingchannel 83 or 85 including cardiac event detection circuitry, e.g., asdescribed in conjunction with FIGS. 5 and 12. Switching circuitry mayinclude a switch array, switch matrix, multiplexer, or any other type ofswitching device suitable to selectively couple components of sensingcircuit 86 to selected electrodes. The cardiac event detection circuitrywithin sensing circuit 86 may include one or more sense amplifiers,filters, rectifiers, threshold detectors, comparators, analog-to-digitalconverters (ADCs), or other analog or digital components as describedfurther in conjunction with FIGS. 5 and 12. A cardiac event sensingthreshold may be automatically adjusted by sensing circuit 86 under thecontrol of control circuit 80, based on timing intervals and sensingthreshold values determined by control circuit 80, stored in memory 82,and/or controlled by hardware of control circuit 80 and/or sensingcircuit 86.

In some examples, sensing circuit 86 includes multiple sensing channels83 and 85 for acquiring cardiac electrical signals from multiple sensingvectors selected from electrodes 24, 26, 28, 30, 31 and housing 15. Eachsensing channel 83 and 85 may be configured to amplify, filter anddigitize the cardiac electrical signal received from selected electrodescoupled to the respective sensing channel to improve the signal qualityfor detecting cardiac events, such as R-waves. For example, each sensingchannel 83 and 85 may include a pre-filter and amplifier for filteringand amplifying a signal received from a selected pair of electrodes. Theresulting raw cardiac electrical signal may be passed from thepre-filter and amplifier to cardiac event detection circuitry in atleast one sensing channel 83 for sensing cardiac events from thereceived cardiac electrical signal in real time. As disclosed herein,sensing channel 83 may be configured to sense cardiac events such asR-waves based on a cardiac event sensing threshold, and second sensingchannel 85 may be configured to pass a digitized cardiac electricalsignal obtained from a different sensing electrode vector to controlcircuit 80 for use in confirming a cardiac event sensed by first sensingchannel 83.

Upon detecting a cardiac event based on a sensing threshold crossing,first sensing channel 83 may produce a sensed event signal, such as anR-wave sensed event signal, that is passed to control circuit 80. Thesensed event signal is used by control circuit 80 to trigger storage ofa time segment of the second cardiac electrical signal forpost-processing and analysis for confirming the R-wave sensed eventsignal as described below, e.g., in conjunction with FIGS. 7 through 9.Memory 82 may be configured to store a predetermined number of cardiacelectrical signal segments in circulating buffers under the control ofcontrol circuit 80, e.g., at least one, two or other number of cardiacelectrical signal segments. Each segment may be written to memory 82over a time interval extending before and after the R-wave sensed eventsignal produced by the first sensing channel 83. Control circuit 80 mayaccess stored cardiac electrical signal segments when confirmation ofR-waves sensed by the first sensing channel 83 is required based on thedetection of a predetermined number of tachyarrhythmia intervals, whichmay precede tachyarrhythmia detection.

The R-wave sensed event signals are also used by control circuit 80 fordetermining RR intervals (RRIs) for detecting tachyarrhythmia anddetermining a need for therapy. An RRI is the time interval betweenconsecutively sensed R-waves and may be determined between consecutiveR-wave sensed event signals received from sensing circuit 86. Forexample, control circuit 80 may include a timing circuit 90 fordetermining RRIs between consecutive R-wave sensed event signalsreceived from sensing circuit 86 and for controlling various timersand/or counters used to control the timing of therapy delivery bytherapy delivery circuit 84. Timing circuit 90 may additionally set timewindows such as morphology template windows, morphology analysis windowsor perform other timing related functions of ICD 14 includingsynchronizing cardioversion shocks or other therapies delivered bytherapy delivery circuit 84 with sensed cardiac events.

Control circuit 80 is also shown to include a tachyarrhythmia detector92 configured to analyze signals received from sensing circuit 86 fordetecting tachyarrhythmia episodes. Tachyarrhythmia detector 92 may beimplemented in control circuit 80 as hardware and/or firmware thatprocesses and analyzes signals received from sensing circuit 86 fordetecting VT and/or VF. In some examples, the timing of R-wave senseevent signals received from sensing circuit 86 is used by timing circuit90 to determine RRIs between sensed event signals. Tachyarrhythmiadetector 92 may include comparators and counters for counting RRIsdetermined by timing circuit 92 that fall into various rate detectionzones for determining a ventricular rate or performing other rate- orinterval-based assessment for detecting and discriminating VT and VF.

For example, tachyarrhythmia detector 92 may compare the RR's determinedby timing circuit 90 to one or more tachyarrhythmia detection intervalzones, such as a tachycardia detection interval zone and a fibrillationdetection interval zone. RR's falling into a detection interval zone arecounted by a respective VT interval counter or VF interval counter andin some cases in a combined VT/VF interval counter included intachyarrhythmia detector 92. When an interval counter reaches adetection threshold, a ventricular tachyarrhythmia may be detected bytachyarrhythmia detector 92. Tachyarrhythmia detector 92 may beconfigured to perform other signal analysis for determining if otherdetection criteria are satisfied before detecting VT or VF, such asR-wave morphology criteria, onset criteria, and noise and oversensingrejection criteria. Examples of other parameters that may be determinedfrom cardiac electrical signals received by sensing circuit 86 fordetermining the status of tachyarrhythmia detection rejection rules thatmay cause withholding to a VT or VF detection are described inconjunction with FIGS. 10, 11 and 13.

To support these additional analyses, sensing circuit 86 may pass adigitized electrocardiogram (ECG) signal to control circuit 80 formorphology analysis performed by tachyarrhythmia detector 92 fordetecting and discriminating heart rhythms. A cardiac electrical signalfrom the selected sensing vector, e.g., from first sensing channel 83and/or the second sensing channel 85, may be passed through a filter andamplifier, provided to a multiplexer and thereafter converted tomulti-bit digital signals by an analog-to-digital converter, allincluded in sensing circuit 86, for storage in memory 82. Memory 82 mayinclude one or more circulating buffers to temporarily store digitalcardiac electrical signal segments for analysis performed by controlcircuit 80 to confirm R-waves sensed by sensing channel 83, determinemorphology matching scores, detect T-wave oversensing, detect noisecontamination, and more as further described below.

Control circuit 80 may be a microprocessor-based controller that employsdigital signal analysis techniques to characterize the digitized signalsstored in memory 82 to recognize and classify the patient's heart rhythmemploying any of numerous signal processing methodologies for analyzingcardiac signals and cardiac event waveforms, e.g., R-waves. Examples ofdevices and algorithms that may be adapted to utilize techniques forR-wave sensing and confirmation and tachyarrhythmia detection describedherein are generally disclosed in U.S. Pat. No. 5,354,316 (Keimel); U.S.Pat. No. 5,545,186 (Olson, et al.); U.S. Pat. No. 6,393,316 (Gillberg etal.); U.S. Pat. No. 7,031,771 (Brown, et al.); U.S. Pat. No. 8,160,684(Ghanem, et al.), and U.S. Pat. No. 8,437,842 (Zhang, et al.), all ofwhich patents are incorporated herein by reference in their entirety.

Therapy delivery circuit 84 includes charging circuitry; one or morecharge storage devices, such as one or more high voltage capacitorsand/or low voltage capacitors, and switching circuitry that controlswhen the capacitor(s) are discharged across a selected pacing electrodevector or CV/DF shock vector. Charging of capacitors to a programmedpulse amplitude and discharging of the capacitors for a programmed pulsewidth may be performed by therapy delivery circuit 84 according tocontrol signals received from control circuit 80. Timing circuit 90 ofcontrol circuit 80 may include various timers or counters that controlwhen ATP or other cardiac pacing pulses are delivered. For example,timing circuit 90 may include programmable digital counters set by amicroprocessor of the control circuit 80 for controlling the basic timeintervals associated with various pacing modes or ATP sequencesdelivered by ICD 14. The microprocessor of control circuit 80 may alsoset the amplitude, pulse width, polarity or other characteristics of thecardiac pacing pulses, which may be based on programmed values stored inmemory 82.

During pacing, escape interval counters within timing circuit 90 arereset upon sensing of R-waves as indicated by signals from sensingcircuit 86. In accordance with the selected mode of pacing, pacingpulses are generated by a pulse output circuit of therapy deliverycircuit 84 when an escape interval counter expires. The pace outputcircuit is coupled to the desired pacing electrodes via a switch matrixfor discharging one or more capacitors across the pacing load. Theescape interval counters are reset upon generation of pacing pulses, andthereby control the basic timing of cardiac pacing functions, includingATP. The durations of the escape intervals are determined by controlcircuit 80 via a data/address bus. The value of the count present in theescape interval counters when reset by sensed R-waves can be used tomeasure RRIs by timing circuit 90 as described above for detecting theoccurrence of a variety of arrhythmias by tachyarrhythmia detector 92.

Memory 82 may include read-only memory (ROM) in which stored programscontrolling the operation of the control circuit 80 reside. Memory 82may further include random access memory (RAM) or other memory devicesconfigured as a number of recirculating buffers capable of holding aseries of measured RRIs, counts or other data for analysis by thetachyarrhythmia detector 92 for predicting or diagnosing an arrhythmia.

In response to the detection of ventricular tachycardia, ATP therapy canbe delivered by loading a regimen from the microprocessor included incontrol circuit 80 into timing circuit 90 according to the type and rateof tachycardia detected. In the event that higher voltage cardioversionor defibrillation pulses are required, e.g., the tachyarrhythmia is VFor the VT is not terminated via the ATP therapy, the control circuit 80activates cardioversion and defibrillation control circuitry included incontrol circuit 80 to initiate charging of the high voltage capacitorsvia a charging circuit, both included in therapy delivery circuit 84,under the control of a high voltage charging control line. The voltageon the high voltage capacitors is monitored via a voltage capacitorline, which is passed to control circuit 80. When the voltage reaches apredetermined value set by control circuit 80, a logic signal isgenerated on a capacitor full line passed to therapy delivery circuit84, terminating charging. The defibrillation or cardioversion pulse isdelivered to the heart under the control of the timing circuit 90 by anoutput circuit of therapy delivery circuit 84 via a control bus. Theoutput circuit determines the electrodes used for delivering thecardioversion or defibrillation pulse and the pulse wave shape. Therapydelivery and control circuitry generally disclosed in any of theabove-incorporated patents may be implemented in ICD 14.

Control parameters utilized by control circuit 80 for detecting cardiacrhythms and controlling therapy delivery may be programmed into memory82 via telemetry circuit 88. Telemetry circuit 88 includes a transceiverand antenna for communicating with external device 40 (shown in FIG. 1A)using RF communication as described above. Under the control of controlcircuit 80, telemetry circuit 88 may receive downlink telemetry from andsend uplink telemetry to external device 40. In some cases, telemetrycircuit 88 may be used to transmit and receive communication signalsto/from another medical device implanted in patient 12.

FIG. 5 is diagram of circuitry included in first sensing channel 83 andsecond sensing channel 85 of sensing circuit 86 according to oneexample. First sensing channel 83 may be selectively coupled viaswitching circuitry (not shown) to a first sensing electrode vectorincluding electrodes carried by extra-cardiovascular lead 16 as shown inFIGS. 1A-2C for receiving a first cardiac electrical signal. Firstsensing channel 83 may be coupled to a sensing electrode vector that isa short bipole, having a relatively shorter inter-electrode distance orspacing than the second electrode vector coupled to second sensingchannel 85. In the example shown, the first sensing electrode vector mayinclude pace/sense electrodes 28 and 30. In other examples, the firstsensing electrode vector coupled to sensing channel 83 may includepace/sense electrodes 30 and 31 and in some cases pace/sense electrodes28 and 31 depending on the inter-electrode spacing and position of thedistal portion 25 of lead 16. In other examples, the first sensingchannel 83 may be selectively coupled to a sensing electrode vectorincluding a defibrillation electrode 24 and/or 26, e.g., a sensingelectrode vector between pace/sense electrode 28 and defibrillationelectrode 24, between pace/sense electrode 30 and either ofdefibrillation electrodes 24 or 26, or between pace/sense electrode 26and 31, for example. In some examples, the first sensing electrodevector may be between defibrillation electrodes 24 and 26.

Sensing circuit 86 includes a second sensing channel 85 that receives asecond cardiac electrical signal from a second sensing vector, forexample from a vector that includes electrode 30 and housing 15, asshown, or a vector that includes electrode 28 and housing 15. Secondsensing channel 85 may be selectively coupled to other sensing electrodevectors, which may form a long bipole having an inter-electrode distanceor spacing that is greater than the sensing electrode vector coupled tofirst sensing channel 83. As described below, the second cardiacelectrical signal received by second sensing channel 85 via a longbipole may be used by control circuit 80 for morphology analysis(including beat morphology analysis, noise rejection and other analyses,for example as described in conjunction with FIG. 10). In otherexamples, any vector selected from the available electrodes, e.g.,electrodes 24, 26, 28, 30 and/or 31 and/or housing 15 may be included ina sensing electrode vector coupled to second sensing channel 85.

The electrical signals developed across input electrodes 28 and 30 ofsensing channel 83 and across input electrodes 30 and 15 of sensingchannel 85 are provided as differential input signals to the pre-filterand pre-amplifiers 62 and 72, respectively. Non-physiological highfrequency and DC signals may be filtered by a low pass or bandpassfilter included in each of pre-filter and pre-amplifiers 62 and 72, andhigh voltage signals may be removed by protection diodes included inpre-filter and pre-amplifiers 62 and 72. Pre-filter and pre-amplifiers62 and 72 may amplify the pre-filtered signal by a gain of between 10and 100, and in one example a gain of 17.5, and may convert thedifferential signal to a single-ended output signal passed toanalog-to-digital converter (ADC) 63 in first sensing channel 83 and toADC 73 in second sensing channel 85. Pre-filters and amplifiers 62 and72 may provide anti-alias filtering and noise reduction prior todigitization.

ADC 63 and ADC 73, respectively, convert the first cardiac electricalsignal from an analog signal to a first digital bit stream and thesecond cardiac electrical signal to a second digital bit stream. In oneexample, ADC 63 and ADC 73 may be sigma-delta converters (SDC), butother types of ADCs may be used. In some examples, the outputs of ADC 63and ADC 73 may be provided to decimators (not shown), which function asdigital low-pass filters that increase the resolution and reduce thesampling rate of the respective first and second cardiac electricalsignals.

In sensing channel 83, the digital output of ADC 63 is passed to filter64 which may be a digital bandpass filter have a bandpass ofapproximately 10 Hz to 30 Hz for passing cardiac electrical signals suchas R-waves typically occurring in this frequency range. The bandpassfiltered signal is passed from filter 64 to rectifier 65 then to R-wavedetector 66. R-wave detector 66 may include an auto-adjusting senseamplifier, comparator and/or other detection circuitry that compares thefiltered and rectified first cardiac electrical signal to an R-wavesensing threshold in real time and produces an R-wave sensed eventsignal 68 when the cardiac electrical signal crosses the R-wave sensingthreshold.

The R-wave sensing threshold may be controlled by sensing circuit 86and/or control circuit 80 to be a multi-level sensing threshold asdisclosed in U.S. patent application Ser. No. 15/142,171 (Cao, et al.,filed on Apr. 29, 2016), incorporated herein by reference in itsentirety. Briefly, the multi-level sensing threshold may have a startingsensing threshold value held for a time interval equal to a tachycardiadetection interval, then drops to a second sensing threshold value helduntil a drop time interval expires, which may be 1 to 2 seconds long.The sensing threshold drops to a minimum sensing threshold after thedrop time interval. The starting sensing threshold value may be thelower of a predetermined percentage of the most recent, preceding sensedR-wave peak amplitude and a maximum sensing threshold limit determinedusing a sensitivity-dependent gain and the programmed sensitivitysetting. In other examples, the R-wave sensing threshold used by R-wavedetector 66 may be set to a starting value based on a preceding R-wavepeak amplitude and decay linearly or exponentially over time untilreaching a minimum sensing threshold. However, the techniques of thisapplication are not limited to a specific behavior of the sensingthreshold. Instead, other automatically adjusted sensing thresholds maybe utilized.

In some examples, the filtered, digitized cardiac electrical signal fromsensing channel 83 (output of filter 64) may be stored in memory 82 forsignal processing by control circuit 80 for use in detectingtachyarrhythmia episodes. In one example, the output of rectifier 64 ispassed to differentiator 67 which determines an Nth order differentialsignal 69 that is passed to memory 82. The differential signal 69 isalso sometimes referred to as a “difference signal” because each samplepoint of the differential signal 69 may be determined as the differencebetween the ith input sample point and a corresponding i-N input samplepoint. Control circuit 80 may retrieve the stored signal from memory 82for performing signal analysis by tachyarrhythmia detector 92 accordingto implemented tachyarrhythmia detection algorithms. For example, aT-wave oversensing algorithm implemented in tachyarrhythmia detector 92may detect evidence of T-wave oversensing from a first orderdifferential signal 69 produced by differentiator 67 as described inU.S. patent application Ser. No. ______, (Atty. Docket No.C00013321.USU1, Cao, et al.), incorporated herein by reference in itsentirety. Other examples of methods for detecting T-wave oversensingusing a differential signal may be performed by tachyarrhythmia detector92 as generally disclosed in U.S. Pat. No. 7,831,304 (Cao, et al.),incorporated herein by reference in its entirety.

The second cardiac electrical signal, digitized by ADC 73, may be passedto filter 74 for bandpass filtering, e.g., from 10 Hz to 30 Hz. In someexamples, sensing channel 85 includes notch filter 76. Notch filter 76may be implemented in firmware or hardware and is provided to attenuate50-60 Hz electrical noise, muscle noise and other electromagneticinterference (EMI) or electrical noise/artifacts in the second cardiacelectrical signal. Cardiac electrical signals acquired usingextra-cardiovascular electrodes as shown, for example in FIGS. 1A-3, maybe more likely to be contaminated by 50-60 Hz electrical noise, musclenoise and other EMI, electrical noise/artifacts than intra-cardiacelectrodes. As such, notch filter 76 may be provided to significantlyattenuate the magnitude of signals in the range of 50-60 Hz with minimumattenuation of signals in the range of approximately 1-30 Hz,corresponding to typical cardiac electrical signal frequencies. Oneexample of a notch filter, designed with minimal computationalrequirements, and its filtering characteristics are described inconjunction with FIG. 6.

The output signal 78 of notch filter 76 may be passed from sensingcircuit 86 to memory 82 under the control of control circuit 80 forstoring segments of the second cardiac electrical signal 78 in temporarybuffers of memory 82. For example, timing circuit 90 of control circuit80 may set a time interval or number of sample points relative to anR-wave sensed event signal 68 received from first sensing channel 83,over which the second cardiac electrical signal 78 is stored in memory82. The buffered, second cardiac electrical signal segment is analyzedby control circuit 80 on a triggered, as needed basis, as described inconjunction with FIGS. 7-13 to confirm R-waves sensed by the firstsensing channel 83.

Notch filter 76 may be implemented as a digital filter for real-timefiltering performed by firmware as part of sensing channel 85 or bycontrol circuit 80 for filtering the buffered digital output of filter74. In some examples, the output of filter 74 of sensing channel 85 maybe stored in memory 82 in time segments defined relative to an R-wavesense event signal 68 prior to filtering by notch filter 76. Whencontrol circuit 80 is triggered to analyze the stored, second cardiacelectrical signal for confirming an R-wave sensed event signal, forexample as described in conjunction with FIGS. 7, 10, 11 and 13, thenotch filter 76 may be applied to the stored segment of the secondcardiac electrical signal before further processing and analysis of thestored segment. In this way, if analysis of the stored signal segment isnot required for confirming an R-wave sensed by first sensing channel83, firmware implemented to perform the operation of notch filter 76need not be executed.

The configuration of sensing channels 83 and 85 is illustrative innature and should not be considered limiting of the techniques describedherein. The sensing channels 83 and 85 of sensing circuit 86 may includemore or fewer components than illustrated and described in FIG. 5. Firstsensing channel 83, however, is configured to detect R-waves in realtime, e.g., in hardware implemented components, from a first cardiacelectrical signal based on crossings of an R-wave sensing threshold bythe first cardiac electrical signal, and second sensing channel 85 isconfigured to provide a second cardiac electrical signal for storage inmemory 82 for post-processing and analysis by control circuit 80 forconfirming R-wave sensed event signals produced by the first sensingchannel 83.

FIG. 6 is a plot 50 of the attenuation characteristics of notch filter76 of the second sensing channel 85. In one example, notch filter 76 isimplemented in firmware as a digital filter. The output of the digitalnotch filter may be determined by firmware implemented in the secondsensing channel 85 according to the equation:

Y(n)=(x(n)+2x(n−2)+x(n−4))/4

where x(n) is the amplitude of the nth sample point of the digitalsignal received by the notch filter 76, x(n−2) is the amplitude of then−2 sample point, and x(n−4) is the amplitude of the n−4 sample pointfor a sampling rate of 256 Hz. Y(n) is the amplitude of the nth samplepoint of the notch-filtered, digital second cardiac electrical signal.The plot 50 of FIG. 6 represents the resulting attenuation of theamplitude Y(n) as a function of frequency. At a frequency of 60 Hz, theattenuation of the magnitude of Y(n) is −40 decibels (dB). At afrequency of 50 Hz, the attenuation is −20 dB, and at 23 Hz, which maybe typical of an R-wave of the cardiac electrical signal, theattenuation is limited to −3 dB. Notch filter 76 may therefore providehighly attenuated 50 and 60 Hz noise, muscle noise, other EMI, and otherelectrical noise/artifacts while passing lower frequency cardiac signalsin the second cardiac electrical signal output of sensing channel 85.Although the notch filter 76 may not attenuate frequencies approachingthe maximum frequency of 128 Hz, filter 74 of second sensing channel 85,which may be a bandpass filter, may adequately reduce the higherfrequency range signal content above 60 Hz.

The sample point numbers indicated in the equation above for determininga notch-filtered signal may be modified as needed when a differentsampling rate other than 256 Hz is used, however, the resultingfrequency response may or may not be the same as that shown in FIG. 6.The notch filter 76 uses minimal computations with only two adds andthree shifts required. In other examples, other digital filters may beused for attenuation of 50 and 60 Hz. For example, for a sampling rateof 256 Hz, a filtered signal Y(n) may be determined asY(n)=(x(n)+x(n−1)+x(n−2)+x(n−3))/4 which has less attenuation at 50 and60 Hz than the frequency response shown in FIG. 6 but acts as alow-pass, notch filter with greater attenuation at higher frequencies(greater than 60 Hz) than the frequency response shown FIG. 6.

FIG. 7 is a flow chart 100 of a method performed by ICD 14 for sensingand confirming R-waves for use in tachyarrhythmia detection according toone example. At blocks 102 and 104, two different sensing electrodevectors are selected by sensing circuit 86 for receiving a first cardiacelectrical signal by a first sensing channel 83 and a second cardiacelectrical signal by a second sensing channel 85. The two sensingelectrode vectors may be selected by switching circuitry included insensing circuit 86 under the control of control circuit 80. In someexamples, the two sensing electrode vectors are programmed by a user andretrieved from memory 82 by control circuit 80 and passed to sensingcircuit 86 as vector selection control signals.

The first sensing vector selected at block 102 for obtaining a firstcardiac electrical signal may be a relatively short bipole, e.g.,between electrodes 28 and 30 or between electrodes 28 and 24 of lead 16or other electrode combinations as described above. The relatively shortbipole may include electrodes that are in relative close proximity toeach other and to the ventricular heart chambers compared to otheravailable sensing electrode pairs. The first sensing vector may be avertical sensing vector (with respect to an upright or standing positionof the patient) or approximately aligned with the cardiac axis formaximizing the amplitude of R-waves in the first cardiac electricalsignal for reliable R-wave sensing.

The second sensing electrode vector used to obtain a second cardiacelectrical signal at block 104 may be a relatively long bipole having aninter-electrode distance that is greater than the first sensingelectrode vector. For example, the second sensing electrode vector maybe selected as the vector between one of the pace sense electrodes 28 or30 and ICD housing 15, one of defibrillation electrodes 24 or 26 andhousing 15 or other combinations of one electrode along the distalportion of the lead 16 and the housing 15. This sensing vector may beorthogonal or almost orthogonal to the first sensing vector in someexamples, but the first and second sensing vectors are not required tobe orthogonal vectors. The second sensing electrode vector may provide arelatively more global or far-field cardiac electrical signal comparedto the first sensing electrode vector. The second cardiac electricalsignal obtained at block 104 may be used for waveform morphologyanalysis by the tachyarrhythmia detector 92 of control circuit 80 and isused for cardiac signal analysis for confirming an R-wave sensed eventsignal produced by first sensing channel 83 of sensing circuit 86.

Sensing circuit 86 may produce an R-wave sensed event signal at block106 in response to the first sensing channel 83 detecting an R-wavesensing threshold crossing by the first cardiac electrical signal. TheR-wave sensed event signal may be passed to control circuit 80. Inresponse to the R-wave sensed event signal, down-going “yes” branch ofblock 106, control circuit 80 is triggered at block 108 to store asegment of the second cardiac electrical signal received from the secondsensing channel 85 (sensing vector 2, block 104) in a circulating bufferof memory 82. A digitized segment of the second cardiac electricalsignal may be 100 to 500 ms long, for example, including sample pointsbefore and after the time of the R-wave sensed event signal, which mayor may not be centered in time on the R-wave sensed event signalreceived from sensing circuit 86. For instance, the segment may extend100 ms after the R-wave sensed event signal and be 200 to 500 ms induration such that the segment extends from about 100 to 400 ms beforethe R-wave sensed event signal to 100 ms after. In other examples, thesegment may be centered on the R-wave sensed event signal or extend agreater number of sample points after the R-wave sensed event signalthan before. In one example, the buffered segment of the cardiacelectrical signal is at least 50 sample points obtained at a samplingrate of 256 Hz, or about 200 ms. In another example, the bufferedsegment is at least 92 sample points, or approximately 360 ms, sampledat 256 Hz and is available for morphology analysis, noise analysis,T-wave oversensing, and/or other analysis performed by tachyarrhythmiadetector 92 for detecting VT or VF. Other analyses of the bufferedsecond cardiac electrical signal that may be performed bytachyarrhythmia detector 92 for detecting VT or VF, or withholdingdetection of VT or VF, are described in conjunction with FIG. 10. Memory82 may be configured to store a predetermined number of second cardiacelectrical segments, e.g., at least 1 and in some cases two or morecardiac electrical signal segments, in circulating buffers such that theoldest segment is overwritten by the newest segment. However, previouslystored segments may never be analyzed for R-wave confirmation beforebeing overwritten if an R-wave confirmation threshold is not reached asdescribed below. In some examples, a single segment of the secondcardiac electrical signal may be stored and if not needed for confirmingan R-wave sensed by the first channel, the segment is overwritten by thenext segment corresponding to the next R-wave sensed event signal.

In addition to buffering a segment of the second cardiac electricalsignal, control circuit 80 responds to the R-wave sensed event signalproduced at block 106 by determining an RRI at block 110 ending with thecurrent R-wave sensed event signal and beginning with the most recentpreceding R-wave sensed event signal. The timing circuit 90 of controlcircuit 80 may pass the RRI timing information to the tachyarrhythmiadetection circuit 92 which adjusts tachyarrhythmia interval counters atblock 112. If the RRI is longer than a tachycardia detection interval(TDI), the tachyarrhythmia interval counters remain unchanged. If theRRI is shorter than the TDI but longer than a fibrillation detectioninterval (FDI), i.e., if the RRI is in a tachycardia detection intervalzone, a VT interval counter is increased at block 112. If the RRI isshorter than or equal to the FDI, a VF interval counter is increased atblock 112. In some examples, a combined VT/VF interval counter isincreased if the RRI is less than the TDI.

After updating the tachyarrhythmia interval counters at block 112,tachyarrhythmia detector 92 compares the counter values to an R-senseconfirmation threshold at block 114 and to VT and VF detectionthresholds at block 132. If a VT or VF detection interval counter hasreached an R-sense confirmation threshold, “yes” branch of block 114,the second cardiac electrical signal from sensing channel 85 is analyzedto confirm the R-wave sensed at block 106 by the first sensing channel83. The R-sense confirmation threshold may be a VT or VF interval countthat is greater than or equal to a count of one or another higher countvalue. Different R-sense confirmation thresholds may be applied to theVT interval counter and the VF interval counter. For example, theR-sense confirmation threshold may be a count of two on the VT intervalcounter and a count of three on the VF interval counter. In otherexamples, the R-sense confirmation threshold is a higher number, forexample five or higher, but may be less than the number of intervalsrequired to detect VT or VF. In addition or alternatively to applying anR-sense confirmation threshold to the individual VT and VF counters, anR-sense confirmation threshold may be applied to a combined VT/VFinterval counter.

If the R-sense confirmation threshold is not reached by any of thetachyarrhythmia interval counters at block 114, the control circuit 80waits for the next R-wave sensed event signal at block 108 to buffer thenext segment of the second cardiac electrical signal. If the R-senseconfirmation threshold is reached at block 114, the control circuit 80determines a maximum amplitude at block 116 of the buffered signalsegment stored for the most recent R-wave sensed event signal. Themaximum amplitude may be determined from a differential signaldetermined from the buffered signal segment. For example an nth-orderdifferential signal may be determined from the buffered signal segmentby determining a difference between the ith and the ith-n signal samplepoints of the buffered signal segment. In one example a 4th orderdifferential signal is determined.

The maximum absolute value of the differential signal is estimated asthe amplitude of the event in the second cardiac electrical signal thatwas sensed as an R-wave from the first cardiac electrical signal. Thetime of the maximum absolute value of the signal is identified as thetime of the event in the second cardiac electrical signal. When theR-wave is not the first R-wave to be confirmed since the R-senseconfirmation threshold was reached, the control circuit 80 determines anamplitude ratio at block 118 as the ratio of the maximum absolute valuedetermined at block 116 to the event amplitude determined from thesecond cardiac electrical signal for the most recently confirmed R-wavesensed event. At block 120, the control circuit 80 determines a timeinterval from the most recent event of the second cardiac electricalsignal confirmed as an R-wave sensed event to the time of the eventdetermined at block 116.

When the R-wave is the first R-wave to be confirmed after the R-senseconfirmation threshold is reached, the first confirmed event on thesecond cardiac electrical signal may be assumed to occur at the sametime as the R-wave sensed event signal with a default maximum amplitude.The default maximum amplitude may be set equal to the amplitude of theR-wave sensed by the first sensing channel 83, a nominal value, e.g., 1millivolt, or a previously determined R-wave amplitude or average R-waveamplitude determined from the second cardiac electrical signal.Alternatively, the maximum absolute amplitude of the differential signaland its time may be identified and stored as initial values used fordetermining an amplitude ratio and time at blocks 118 and 120 for thenext R-wave to be confirmed. In other examples, an amplitude ratio maybe determined for the first R-wave to be confirmed after the R-senseconfirmation threshold is reached using a previously determined R-waveamplitude, e.g., from a prior time that the R-sense confirmationthreshold was reached or a default R-wave amplitude. The first R-wavemay be confirmed based on this amplitude ratio and/or time since thepreceding R-wave sensed event signal.

At block 122, the control circuit 80 determines a ratio threshold to beapplied to the amplitude ratio based on the time interval determined atblock 120. In one example, the ratio threshold is retrieved from alook-up table stored in memory. In other examples, the ratio thresholdmay be computed as a function of the time interval determined at block120. The ratio threshold may be a variable threshold that decreases asthe time interval since the most recent confirmed R-wave increases. Assuch, the time interval determined at block 120 is used to determinewhat ratio threshold should be applied to the amplitude ratio determinedat block 118 for confirming the R-wave sensed by first sensing channel83. The ratio threshold may decrease in a linear, exponential orstepwise manner, or a combination thereof. For instance, the ratiothreshold may decrease with a continuous slope or decay rate over someportions of time since the most recent confirmed R-wave and may be heldconstant over other portions of time since the most recent confirmedR-wave. An example of a time-varying ratio threshold and method fordetermining the ratio threshold at block 122 is described in conjunctionwith FIGS. 8 and 9.

At block 124, control circuit 80 compares the ratio threshold determinedat block 122 to the amplitude ratio determined at block 118. If theamplitude ratio is equal to or greater than the ratio threshold, theR-wave sensed event is confirmed at block 126. If the amplitude ratio isless than the ratio threshold, the R-wave sensed event is not confirmedat block 128. The event may be an oversensed T-wave, P-wave, musclenoise, electromagnetic interference or other or non-cardiac electricalnoise that has been oversensed by the first sensing channel 83.

At block 130 the control circuit 80 adjusts an unconfirmed beat counter.If the R-wave sensed event is not confirmed, the unconfirmed beatcounter is increased by one count. If the R-wave sensed event isconfirmed at block 126, the unconfirmed beat counter may kept at itscurrent value or decreased. In some examples, the unconfirmed beatcounter tracks how many out of the most recent predetermined number ofconsecutive R-wave sensed event signals produced by first sensingchannel 83 are not confirmed in an x out of y manner. For example, theunconfirmed beat counter may track how many out of the most recent 12R-wave sensed event signals are not confirmed to be R-waves based on theamplitude ratio comparison made at block 124.

In addition to counting how many beats are unconfirmed at block 130,data relating to the most recent n events analyzed by control circuit 80may be stored in a rolling buffer. For example, data may be stored forthe most recent twelve events analyzed for confirming an R-wave sensedevent signal. The stored data may include the event amplitude, theamplitude ratio, the event timing, the time interval since the mostrecent confirmed event, and whether the event was confirmed or notconfirmed.

While the R-wave sensed event signal is either confirmed or notconfirmed based on an amplitude ratio determined from the second cardiacelectrical signal according to the example of FIG. 7, it is recognizedthat other features of the second cardiac signal may be compared toR-wave confirmation criteria in addition to or instead of the eventamplitude as described above. For example, a peak slew rate, an eventarea, an event signal width, or other features of the buffered cardiacelectrical signal segment may be compared to respective thresholds forconfirming the event as being an R-wave. The thresholds may be definedas a minimum ratio of the feature relative to an analogous feature ofthe most recent preceding event confirmed to be an R-wave or may bethresholds compared directly to features determined from the bufferedcardiac electrical signal segment independent of preceding events.

If any of the tachyarrhythmia interval counters adjusted at block 112reach a number of intervals to detect (NID) tachyarrhythmia, asdetermined at block 132, tachyarrhythmia detector 92 of control circuit80 determines whether a rejection rule is satisfied at block 134 beforedetecting the tachyarrhythmia. In one example, the NID required todetect VT may be a count of 16 VT intervals, which are RRIs that fallinto a predetermined VT interval range or zone. The NID to detect VF maybe a count of 30 VF intervals out of the last 40 RRIs where the VFintervals are RRIs that fall into a predetermined VF interval range orzone. If an NID is reached, one or more rejection rules may be appliedfor rejecting a VT or VF detection based on RRI counts satisfying theNID. Various rejection rules are described below, e.g., in conjunctionwith FIGS. 10, 11 and 13. At least one rejection rule may relate to thenumber of R-waves sensed by the first sensing channel 83 that were notconfirmed by the analysis of the second cardiac electrical signal.Another rejection rule may relate to the detection of noisy signalsegments based on gross morphology analysis as described in conjunctionwith FIGS. 14 through 19.

For instance, the unconfirmed beat counter updated at block 130 may becompared to a rejection rule criterion at block 134. The rejection rulecriterion may be a rejection threshold requiring that at least x of yevents are not confirmed R-waves. For example, if at least 3, at least4, at least 6 or other predetermined number of the most recent 12 events(or other predetermined number of events) analyzed for confirming anR-wave sensed event signal are not confirmed R-waves, the rejection ruleis satisfied, “yes” branch of block 134. The pending VT or VF detectionbased on the NID being reached at block 132 is withheld at block 140,and no anti-tachyarrhythmia therapy is delivered.

If all rejection rules are not satisfied, “no” branch of block 134, thepending detection of the VT or VF episode is not withheld. VT or VF isdetected at block 136 based on the respective VT or VF interval counterreaching a corresponding NID. Control circuit 80 controls therapydelivery circuit 84 to deliver an appropriate anti-tachyarrhythmiatherapy, e.g., ATP or a cardioversion/defibrillation shock, according toprogrammed therapy control parameters.

FIG. 8 is a diagram of a filtered, second cardiac electrical signal 200and an amplitude ratio threshold 210 that may be applied to theamplitude ratio determined from the second cardiac electrical signal atblock 118 of FIG. 7 for confirming an R-wave sensed event signal fromthe first sensing channel 83. The amplitude ratio is not a sensingthreshold that is compared to the second cardiac electrical signal 200in real time. The first sensing channel 83 may operate by sensing anR-wave when the first cardiac electrical signal crosses an R-waveamplitude sensing threshold defined in mV. The second cardiac electricalsignal provided to control circuit 80 by the second sensing channel 85,however, is not compared to an amplitude sensing threshold in real timeas it is acquired. Rather, as described in conjunction with FIG. 7, ifthe first sensing channel 83 produces an R-wave sensed event signal, thesecond cardiac electrical signal is buffered in memory 82 and if atachyarrhythmia interval counter reaches an R-wave confirmationthreshold, the buffered signal is post-processed to determine if theratio of a maximum amplitude determined from the buffered signal segmentto the maximum amplitude determined from a preceding confirmed R-wave ofthe second cardiac electrical signal reaches or exceeds the ratiothreshold 210.

The ratio threshold 210 is shown relative to the second cardiacelectrical signal 200 because the ratio threshold 210 is not a fixedvalue but varies over time. Ratio threshold 210 decreases as the timesince the confirmed R-wave 202 increases. This time-variant ratiothreshold is why the control circuit 80 determines the time since thepreceding confirmed R-wave sensed event at block 120 of FIG. 7 in orderto determine the value of the ratio threshold at block 122 that isapplied to the amplitude ratio for confirming or not confirming theR-wave sensed event signal.

Cardiac electrical signal 200 may be produced by the second sensingchannel 85 by filtering, amplifying and digitizing the cardiacelectrical signal received by the second sensing electrode vector. Whilesignal 200 is shown conceptually as having only positive-going waveformsit is to be understood that signal 200 may have positive- andnegative-going portions and need not be a rectified signal. The absolutevalue of the maximum peak amplitude, positive or negative, may bedetermined from the stored second cardiac electrical signal segment atblock 116 of FIG. 7. Cardiac electrical signal 200 includes an R-wave202, a T-wave 204, a P-wave 206, and a subsequent R-wave 240. R-wave 202represents a confirmed event occurring at time point 205. Time point 205is the sample point of the maximum absolute value of the differentialsignal determined in response to an R-wave sense event signal 250 asdescribed above in conjunction with FIG. 7.

If an R-wave sensed event signal occurs during a blanking interval 214following time point 205 of a preceding confirmed R-wave 202, the newR-wave sensed event is not confirmed. Sensing channel 83 may have sensedthe same R-wave 202 twice or sensed non-cardiac electrical noise as anR-wave.

After the blanking interval, the ratio threshold 210, at a time pointcorresponding to the expiration of blanking interval 214, is equal to astarting value 220 which may be set to 0.6 in one example, but may rangebetween 0.4 and 0.7 in other examples. In one implementation, the ratiothreshold 210 is stored in a look-up table and retrieved from memory 82by control circuit 80 for comparison to an amplitude ratio determined inresponse to an R-wave sensed event signal from the first sensingchannel.

FIG. 9 is an example of a look-up table 300 of ratio threshold values304 that may be stored in memory 82 for respective event time intervals,which may be stored as corresponding sample point numbers 302. If theblanking interval 214 is approximately 150 ms, the first sample point atwhich a maximum amplitude may be determined as an event time point is atsample point 38 when the sampling rate is 256 Hz. In other examples,blanking interval 214 may be longer or shorter than 150 ms and the firstsample point number stored in look-up table 300 will correspond to thesample point number at which the blanking interval 214 expires after theconfirmed event time point 205, considered to “zero” sample point.

The ratio threshold is stored for the first sample point number entry asbeing the starting ratio threshold value 220, which is 0.6 in thisexample. If the control circuit 80 receives an R-wave sensed eventsignal from the first sensing channel 83, and a detection intervalcounter is equal to or greater than the R-wave confirmation threshold, amaximum event amplitude and event time is determined from the buffered,second cardiac electrical signal. The event time may be determined asthe sample point number since the event time 205 of the most recentconfirmed R-wave 202. If the event time is determined to be sample pointnumber 38, control circuit 80 retrieves the ratio threshold, 0.6 in thisexample, stored in the look-up table 300 for the sample point number 38.This ratio threshold value is applied to the amplitude ratio determinedfrom the maximum amplitude of the buffered, second cardiac electricalsignal to the maximum amplitude determined from the most recentconfirmed R-wave 202.

Referring again to FIG. 8, the ratio threshold 210 is shown to decreaseat a constant decay rate 222 until the expiration of a first timeinterval 216. Time interval 216 may be defined to start at the timepoint 205 of the confirmed event 202 or start upon expiration ofblanking interval 205. Time interval 216 may extend for up to 1 secondfrom the time point 205 of the most recent confirmed R-wave 202. Thedecay rate 222 may, in one example, be approximately 0.3/second so thatif time interval 216 is approximately 1 second, ratio threshold value224 is 0.3 when the starting ratio threshold value 220 is 0.6.

Beginning at the expiration of time interval 216, ratio threshold 210 isheld at a constant value 224 until a second time interval 218 expires.The constant value 224 is a ratio of approximately 1/3 (0.3) in oneexample but may be between 1/5 (0.2) and 1/2 (0.5) in other examples.Value 224 may be held for up to 500 ms after time interval 216 expires(for a total time interval 218 of up to 1.5 seconds). This change fromthe decay rate 222 to the constant value 224 is reflected in look-uptable 300 as the ratio threshold 0.3 starting at sample point number 256extending through sample point number 383.

At the expiration of time interval 218, the ratio threshold 210 dropsstepwise to an intermediate ratio threshold value 226 then decays at aconstant rate 228 until it reaches a minimum ratio threshold 230. Thestep drop from constant value 224 may be a drop to a ratio threshold ofapproximately 1/6 to 1/4. In one example, the ratio threshold drops fromapproximately 1/3 (0.3) to an intermediate ratio threshold of 1/5 (0.2)at 0.5 seconds after the expiration of time interval 216. This change isreflected in look-up table 300 as the ratio threshold of 0.2 at samplepoint 384 (0.5 seconds after sample point 256).

The second decay rate 228 may be the same as decay rate 222 or a slowerdecay rate such that ratio threshold 210 reaches the minimum ratiothreshold 230, e.g., 1/32 (0.03), 1/64 (0.015) or other predeterminedminimum ratio, approximately 2.5 seconds (sample point number 640) afterthe time point 205 of the preceding confirmed R-wave 202. The behaviorof ratio threshold 210 moving forward in time from confirmed R-wave 202is captured in look-up table 300 (FIG. 9). For example, at the exampledecay rate 222 of 0.3/second, the ratio threshold is 0.587 at samplepoint number 48, and so on.

The values recited here and reflected in look-up table 300 for ratiothreshold values 220, 224, and 226 and 230 and time intervals 216 and218 are illustrative in nature; other values less than or greater thanthe recited values may be used to implement a time-varying ratioamplitude for use in confirming an R-wave sensed event. The values forthe ratio thresholds and time intervals used to control changes from oneratio threshold value to another or a decay rate and total decayinterval will depend in part on the sampling rate, which is 256 Hz inthe examples provided but may be greater than or less than 256 Hz inother examples.

Referring again to FIG. 8, if an R-wave sensed event signal 252 isproduced by first sensing channel 83, control circuit 80 is triggered tostore a time segment 254 of the second cardiac electrical signal 200 inmemory 80. Time segment 254 may be 360 ms in one example, and may bebetween 300 ms and 500 ms in other examples. If a VT or VF or combinedVT/VF interval counter has reached an R-wave confirmation threshold,control circuit 80 determines a maximum amplitude from the bufferedcardiac signal time segment. As described above, the maximum amplitudemay be the maximum absolute value of an x-order differential signaldetermined from the second cardiac electrical signal 200. The maximumamplitude may be determined from a portion of the stored cardiac signaltime segment. For example, the maximum amplitude may be determined froma segment 255 that is a sub-segment or portion of the stored timesegment 254. Segment 255 may be approximately 50 to 300 ms long, e.g.,200 ms long, when the total time segment 254 is 360 ms to 500 ms long.The segment 255 may be defined relative to the time the R-wave sensedevent signal 252 is received.

The sample point number 197 at which the maximum amplitude within thetime segment 255 occurs represents the number of sample points since theevent time 205 (sample point number zero) of the most recent confirmedR-wave 202. The sample point number 197 is determined as the event timeof the maximum amplitude of cardiac signal time segment 255. Controlcircuit 80 uses this sample point number to look up the correspondingratio threshold 304 in look-up table 300. For the sake of example, themaximum amplitude during time segment 255 obtained in response to R-wavesensed event signal 252 may occur at sample point number 197approximately 0.77 seconds after event time point 205. The stored ratiothreshold for sample point number 197 may be approximately 0.4 for adecay rate 222 of approximately 0.3/second (or 0.0012 per sample point)from the starting value 220, which is 0.6 beginning at sample pointnumber 38 in this example. If the amplitude ratio of the maximumamplitude determined at sample point number 197 during time segment 255to the maximum amplitude determined for confirmed R-wave 202 is greaterthan or equal to 0.4, R-wave sensed event 252 is confirmed. In thisexample, the cardiac electrical signal has a low, baseline amplitudeduring interval 255, and as such the R-wave sensed event signal 252 isnot confirmed. Control circuit 80 increases the unconfirmed eventcounter as described in conjunction with FIG. 7.

Similarly, control circuit 80 may receive R-wave sensed event signal 256and determine a maximum amplitude during time segment 259, definedrelative to R-wave sensed event signal 256, of the buffered cardiacelectrical signal segment 258. The event time sample point number 403 atwhich the maximum amplitude occurs since event time 205 is used to lookup the ratio threshold from look up table 300. In this case, theamplitude ratio determined from the buffered, second cardiac electricalsignal during time segment 259 exceeds the ratio threshold 210 at theevent time sample point number 403 of the maximum amplitude during timesegment 259, which corresponds to R-wave 240. R-wave sensed event signal256 is confirmed by control circuit 80. In this way, the second cardiacelectrical signal from sensing channel 85 is analyzed only when anR-wave sensed event confirmation condition is met, e.g., atachyarrhythmia interval counter is active and has reached a thresholdcount, which may be less than a required number of intervals to detect aVT or VF episode. The R-wave sensed event of the first sensing channelis confirmed based on post-processing of the buffered, second cardiacelectrical signal.

FIG. 10 is a flow chart 400 of a method for detecting tachyarrhythmia byICD 14 according to another example. Operations performed at blocks102-114, 132, 136, 138 and 140 in flow chart 400 may generallycorrespond to identically-numbered blocks shown in FIG. 7 and describedabove. At blocks 102 and 104, two different sensing electrode vectorsare selected by sensing circuit 86 for receiving a first cardiacelectrical signal by first sensing channel 83 and a second cardiacelectrical signal by second sensing channel 85 as described above inconjunction with FIGS. 5 and 7.

Sensing circuit 86 may produce an R-wave sensed event signal at block106 in response to the first sensing channel 83 detecting an R-wavesensing threshold crossing by the first cardiac electrical signal. TheR-wave sensed event signal may be passed to control circuit 80. Inresponse to the R-wave sensed event signal, control circuit 80 istriggered at block 108 to store a segment of the second cardiacelectrical signal received from the second sensing channel 85 in acirculating buffer of memory 82. A digitized segment of the secondcardiac electrical signal, which may be defined in time relative to thetime of the R-wave sensed event signal received from sensing circuit 86,and may be 100 to 500 ms long, for example. In one example, the bufferedsegment of the cardiac electrical signal is at least 92 sample pointsobtained at a sampling rate of 256 Hz, or approximately 360 ms, of which68 sample points may precede and include the sample point at which theR-wave sensed event signal was received and 24 sample points may extendafter the sample point at which the R-wave sensed event signal wasreceived.

In addition to buffering a segment of the second cardiac electricalsignal, control circuit 80 responds to the R-wave sensed event signalproduced at block 106 by determining an RRI at block 110 ending with thecurrent R-wave sensed event signal and beginning with the most recentpreceding R-wave sensed event signal. The timing circuit 90 of controlcircuit 80 may pass the RRI timing information to the tachyarrhythmiadetection circuit 92 which adjusts tachyarrhythmia detection counters atblock 112 as described above in conjunction with FIG. 7.

After updating the VT and VF interval counters at block 112,tachyarrhythmia detector 92 compares the interval counter values to anR-sense confirmation threshold at block 114 and to VT and VF NIDdetection thresholds at block 132. If a VT or VF interval counter hasreached an R-sense confirmation threshold, “yes” branch of block 114,the second cardiac electrical signal from sensing channel 85 is analyzedto confirm the R-wave sensed at block 106 by the first sensing channel83. The R-sense confirmation threshold is a count of two on the VTinterval counter and a count of 3 on the VF interval counter in oneexample. Other examples are given above in conjunction with FIG. 7.

If the R-sense confirmation threshold is not reached by any of theinterval counters at block 114, the control circuit 80 waits for thenext R-wave sensed event signal at block 108 to buffer the next segmentof the second cardiac electrical signal. In some cases, the oldestbuffered cardiac signal segment may be overwritten by the next cardiacsignal segment without ever being analyzed for confirming an R-wave, oranalyzed for any other purpose, since analysis of the buffered cardiacsignal segment is not required if the VT and VF interval counters areinactive (at a count of zero) or remain below the R-sense confirmationthreshold.

If an R-sense confirmation threshold is reached at block 114, thecontrol circuit 80 applies a notch filter to the stored, second cardiacelectrical signal segment at block 416. The notch filter applied atblock 416 may correspond to the filter described in conjunction withFIG. 6. The notch filter significantly attenuates 50-60 Hz electricalnoise, muscle noise, other EMI, and other noise/artifacts in the stored,second cardiac electrical signal segment. Using the notch filteredsegment, control circuit 80 performs multiple analyses on the segment todetermine if any rejection rules are satisfied. As described below, if arejection rule is satisfied, a pending VT or VF episode detection madebased on an NID threshold being reached at block 132 may be withheld.

As described in conjunction with FIG. 7, an amplitude ratio may bedetermined at block 418 for confirming the R-wave sensed event signalthat triggered the buffering of the currently stored, second cardiacelectrical signal segment. Determinations made at block 418 may includethe operations performed at blocks 116, 118 and 120 of FIG. 7. Theamplitude ratio is used to update an R-wave confirmation rejection ruleat block 428.

The maximum peak amplitude used to determine the amplitude ratio may bedetermined from a portion of the stored cardiac signal segment at block418. For example if a 360 ms or 500 ms segment is stored at block 108,only a 200 ms segment, e.g., approximately 52 sample points sampled at256 Hz, which may be centered in time on the R-wave sensed event signalmay be analyzed for determining the amplitude ratio at block 418. Alonger signal segment may be stored at block 108 than required fordetermining the amplitude ratio at block 418 so that a longer segment isavailable for other signal analysis procedures performed bytachyarrhythmia detector 92 as described below, e.g., for determining abaseline noise parameter and other gross morphology parameters fordetecting noisy signal segments as described in conjunction with FIGS.14 through 19.

Control circuit 80 may determine an event interval at block 418 as thetime interval or number of sample points from the maximum peak amplitudeto the preceding confirmed R-wave sensed event, when the current R-wavesensed event signal is not the first one being confirmed since theR-sense confirmation threshold was reached at block 114. At block 428,control circuit 80 may compare the amplitude ratio to a ratio threshold,which may be retrieved from a look-up table stored in memory 82 usingthe determined event interval as described in conjunction with FIGS. 8and 9. If the amplitude ratio is greater than the ratio threshold, thesensed R-wave is confirmed. If the amplitude ratio is less than theratio threshold, the sensed R-wave is not confirmed.

An X of Y unconfirmed beat counter may be updated by tachyarrhythmiadetector 92 at block 428 to reflect the number of R-wave sensed eventsignals that are not confirmed out of the most recent Y R-wave sensedevent signals. For example, the X of Y counter may count how manyR-waves are not confirmed to be R-waves out of the most recent 12 R-wavesensed event signals. If the X of Y count reaches a rejection threshold,e.g., if at least 3, 4, 5 or another predetermined number out of 12R-wave sensed event signals are not confirmed to be R-waves, the R-waverejection rule for withholding tachyarrhythmia detection is satisfied. Aflag or logic value may set by control circuit 80 to indicate the R-waverejection rule is satisfied. Updating the R-wave rejection rule at block428 may include operations described in conjunction with FIG. 7 forblocks 122, 124, 126, 128, 130 and 134.

At blocks 420, 422, 424 and 426, other cardiac signal parameters may bedetermined from the notch-filtered, cardiac signal segment for updatingthe status of other tachyarrhythmia detection rejection rules atrespective blocks 430, 432, 434 and 436. In some examples, a digitizedcardiac electrical signal from first sensing channel 83 may be analyzedand used in updating the status of a tachyarrhythmia detection withholdrule. For example, the notch filtered, cardiac electrical signal fromthe second sensing channel 85 may be analyzed at blocks 420, 422 and 426for updating a gross morphology rejection rule, a beat morphologyrejection rule and a noise rejection rule at blocks 430, 432, and 436,respectively. The differential signal 69 (see FIG. 5) from the firstsensing channel 83 may be analyzed at block 424 for updating the T-waveoversensing (TWOS) rule at block 434.

At block 420 one or more gross morphology parameters are determined fromthe notch-filtered, second cardiac signal segment. Gross morphologyparameters may include, but are not limited to, a low slope content, anoise pulse count, a normalized rectified amplitude, a maximum signalwidth, or other noise metrics. Examples of gross morphology parametersthat may be determined at block 420 are described below in conjunctionwith FIGS. 15, 16 and 17. Other examples of gross morphology parametersthat may be determined are generally disclosed in the above-incorporatedU.S. Pat. No. 7,761,150 (Ghanem, et al.) and U.S. Pat. No. 8,437,842(Zhang, et al.). The gross morphology parameters may be determined usingthe entire second cardiac signal segment stored at block 108 or aportion of the stored segment. In one example, at least 92 samplepoints, approximately 360 ms, are analyzed for determining the grossmorphology parameters, which may be a portion of or the entire storedsegment. The portion of the signal segment analyzed for determininggross morphology metrics extends beyond an expected QRS signal width sothat at least a portion of the segment being analyzed corresponds to anexpected baseline portion. In this way, at least one gross morphologyparameter determined, such as the noise pulse count described inconjunction with FIG. 15, is correlated to non-cardiac signal noise thatmay be occurring during the baseline portion, such as non-cardiacmyoelectric noise or electromagnetic interference (EMI).

The gross morphology parameters are used at block 430 to update thestatus of a gross morphology rejection rule. Gross morphology parametersare determined from the signal segment for determining whether thesignal segment, and therefore the second cardiac electrical signal, isnoisy. A noisy segment may include multiple noise signal pulses that areevidence of non-cardiac muscle (myoelectric) noise, EMI, or othernon-cardiac noise signals. A noisy signal segment may be an indicationthat an R-wave sensed event signal during the noisy signal segment isnot reliable for contributing to a VT or VF detection. The grossmorphology analysis may therefore analyze signal segment sample pointsthat are not limited to the R-wave but may extend along an expectedbaseline portion of the segment prior to the R-wave sensed event signalthat triggers the storage of the signal segment. By performing signalmorphology analysis that includes expected baseline portions, which maybe referred to as “gross morphology analysis” since it involves analysisof an entire signal segment or an extended portion of the signal segmentthat extends beyond just an expected R-wave or QRS waveform, noisepulses may be identified and detected to enable detection of noisysignal segments.

Criteria or thresholds may be applied to each gross morphology parameterdetermined, and the gross morphology rejection rule may be satisfiedwhen a required number of the gross morphology parameters meet thecriteria or threshold applied to the respective parameter. For exampleif at least two out of three gross morphology parameters satisfy noisedetection criteria, the gross morphology rejection rule is satisfied.Control circuit 80 may set a flag or logic signal indicating so at block430. Methods for determining gross morphology parameters and whether ornot the gross morphology rejection rule is satisfied are described belowin conjunction with FIGS. 14 through 19.

At block 422 a morphology matching score is determined from the stored,second cardiac electrical signal segment. The morphology matching scoremay be determined by performing wavelet transform or other morphologymatching analysis on a portion of the stored segment, e.g., on at least48 signal sample points or about 190 ms, and may be performed using thenotch filtered signal produced at block 416. The morphology matchinganalysis may include aligning a selected portion of the stored segmentwith a previously-determined known R-wave template and determining amorphology matching score. The morphology matching score may have apossible range of values from 0 to 100 and indicates how well themorphology of the second cardiac signal segment matches the known R-wavetemplate. A wavelet transform method as generally disclosed in U.S. Pat.No. 6,393,316 (Gillberg et al.) is one example of a morphology matchingmethod that may be performed at block 422 for determining a matchingscore.

Other morphology matching methods that may be implemented bytachyarrhythmia detector 92 may compare the wave shape, amplitudes,slopes, inflection time points, number of peaks, or other features ofthe stored second cardiac electrical signal to a known R-wave template.More specifically, waveform duration or width, waveform polarity,waveform positive-going slope, waveform negative-going slope, and/orother waveform features may be used alone or in combination tocharacterize the similarity between the unknown waveform and a knownR-wave template. Morphology matching methods may use one or acombination of two or more morphology features of the stored secondcardiac electrical signal for determining a match to a known R-wavetemplate. A posture-independent method for determining a morphologymatch score may be performed that includes generatingposture-independent R-wave templates for use in template matching asgenerally disclosed in pre-grant U.S. Pat. Publication No. 2016/0022166(Stadler, et al.), incorporated herein by reference in its entirety.Other beat morphology matching techniques that may be used at block 422are generally disclosed in U.S. Pat. No. 8,825,145 (Zhang, et al.) andU.S. Pat. No. 8,983,586 (Zhang et al.), both incorporated herein byreference in their entirety.

The morphology matching score is used at block 432 by tachyarrhythmiadetector 92 to update a beat morphology rejection rule. The beatmorphology rejection rule may be satisfied when a minimum number ofmorphology match scores out of a predetermined number of most recentmorphology match scores exceed a match score threshold in one example.For example, if at least three out of 8 of the most recent morphologymatch scores exceed a match score threshold of 50, 60, 70 or other scorethreshold, the beat morphology rejection rule is satisfied. A relativelyhigh match score, exceeding a selected match score threshold, indicatesthe unknown beat matches the known R-wave template and is therefore anormal R-wave rather than a VT or VF beat. As such, when a thresholdnumber of the most recent morphology match scores are determined to benormal R-waves, the beat morphology rejection rule is satisfied, andcontrol circuit 80 may set a flag or logic signal indicating so.

The beat morphology rejection rule differs from the gross morphologyrejection rule in that the beat morphology rejection rule is based on ananalysis of the suspected R-wave signal to determine how likely it is tobe normal R-wave. In other words the beat morphology rejection rule isapplied to withhold a VT or VF detection based on verifying normalventricular beats (or at least identifying beats that are not likely VTor VF beats). The gross morphology rejection rule is based on ananalysis of a longer time interval of the signal segment than the beatmorphology rejection rule so that a signal morphology parametercorrelated to non-cardiac noise pulses, e.g., during the baselineportion of the second cardiac electrical signal may be detected. Thegross morphology rejection rule is applied to withhold a VT or VFdetection based on detecting noisy signal segments that may beinterfering with reliable VT or VF detection or leading to a false VT orVF detection.

At block 424, TWOS parameters are determined from a stored, digitizedcardiac electrical signal. In some cases, the TWOS parameters aredetermined from a first order differential signal 69 received from firstsensing channel 83 as described in conjunction with FIG. 5 above. Thefirst order differential signal is determined by subtracting theamplitude of the n−1 sample point from the nth sample point.Alternatively or additionally, the second cardiac electrical signal fromsensing channel 85, before or after notch filtering, may be used fordetermining TWOS parameters for detecting TWOS based on morphologyanalysis of the stored cardiac signal, e.g., as generally disclosed inthe above-incorporated U.S. Pat. Application No. 62/367,221 (Atty.Docket No. C00013321.USP1, Cao, et al.). Tachyarrhythmia detector 92 maybe configured to execute T-wave oversensing rejection algorithms bydetermining a differential filtered cardiac electrical signal and TWOSparameters as generally disclosed in the above-incorporated U.S. Pat.No. 7,831,304 (Cao, et al). Other aspects of detecting TWOS that may beused for determining TWOS parameters from the either the first and/orsecond cardiac electrical signal are generally disclosed in U.S. Pat.No. 8,886,296 (Patel, et al.) and U.S. Pat. No. 8,914,106 (Charlton, etal.), both incorporated herein by reference in their entirety.

At block 434, the TWOS parameter(s) determined for the currently storedcardiac signal segment are used by the tachyarrhythmia detector 92 toupdate the status of a TWOS rejection rule as being either satisfied orunsatisfied. For example, if one or more TWOS parameters indicate theR-wave sensed event signal produced by the first sensing channel 83 islikely to be an oversensed T-wave, a TWOS event counter may be updatedat block 434. If the TWOS event counter reaches a threshold, the TWOSrejection rule is satisfied. Control circuit 80 may set a flag or logicsignal indicating when the TWOS rejection rule is satisfied.

Other noise parameters may be determined at block 426 to identifyoversensing due to noise artifacts. The noise parameters determined atblock 426 may include determining peak amplitudes from thenotch-filtered second cardiac electrical signal segment. All or aportion of the stored signal segment may be used for determining one ormore amplitude peaks. In other examples, the first cardiac electricalsignal segment may undergo notch filtering and be used for determiningnoise parameters at bock 426. The peak amplitudes determined at block426 may include the maximum peak amplitude determined at block 418 foruse in determining the amplitude ratio. The maximum peak amplitudes forone or more stored cardiac signal segments are compared to noisedetection criteria for determining whether the noise rejection rule issatisfied at block 436. Control circuit 80 sets a flag or logic signalto indicate the status of the noise rejection rule at block 436. Methodsfor determining noise parameters at block 426 and updating a noiserejection rule at block 436 are generally disclosed in U.S. Pat.Application No. 62/367,170, (Greenhut, et al., Atty. Docket No.C000013169.USP1), incorporated herein by reference in its entirety.

After adjusting the VT and VF interval counters at block 112, thetachyarrhythmia detector 92 compares the interval counters to VT and VFNID detection thresholds at block 132. If the NID has been reached byeither the VT or VF interval counter, tachyarrhythmia detector 92 checksthe status of the rejection rules at block 440. If rejection criteriaare satisfied at block 440, “yes” branch of block 440, based on thestatus of one or more rejection rules, the VT or VF detection based onRRI analysis at blocks 110, 112, and 132 is withheld at block 140. No VTor VF therapy is delivered. The process returns to block 110 todetermine the next RRI upon receiving the next R-wave sensed eventsignal from sensing channel 83.

If the rejection criteria are not satisfied, “no” branch of block 440,the VT or VF episode is detected at block 136 according to which VT orVF interval counter reached its respective NID threshold. Controlcircuit 80 controls therapy delivery circuit 84 to deliver a therapy atblock 138 according to the type of episode detected and programmedtherapy delivery control parameters.

In some examples, the rejection criteria applied at block 440 requireonly a single rejection rule be satisfied in order to cause thetachyarrhythmia detector 92 to withhold a VT or VF detection. In otherexamples, two or more rejection rules may be required to be satisfiedbefore an RRI-based VT or VF detection is withheld. In still otherexamples, one rejection rule may be linked with another rejection rulein order to have rejection criteria satisfied at block 440. Forinstance, the R-wave confirmation rejection rule may only be used tosatisfy the rejection criteria when the gross morphology rejection ruleis also satisfied. In this case, the R-wave confirmation rejection rulealone may not be used to satisfy the rejection criteria at block 440.The gross morphology rejection rule may be used only with the R-waveconfirmation rejection rule, alone or in combination with another ruleto satisfy the rejection criteria.

The rejection rules updated at blocks 428 through 436 may beprogrammably enabled or disabled by a user using external device 40.Control circuit 80 may determine which parameters are determined atblocks 418 through 426 as required for updating the status of only therejection rules that are enabled or programmed “ON.”

FIG. 11 is a flow chart 500 of a method for detecting tachyarrhythmia byICD 14 according to another example. In the examples of FIG. 7 and FIG.10, the number of RR's determined from the first sensing electrodevector 102 that fall into a respective VT or VF interval range or zoneare tracked by respective VT and VF interval counters. The counts of theVT and VF interval counters are compared to respective VT and VF NIDthresholds at block 132. Identically-numbered blocks in FIG. 11correspond to like-numbered blocks shown in FIGS. 7 and 10 as describedabove.

In other examples, the second cardiac electrical signal received by thesecond sensing channel 85 from the second electrode vector at block 104may also be used for determining RR's and determining whether an NIDthreshold is reached at decision block 438. Tachyarrhythmia detector 92may include second VT and VF interval counters for counting RR'sdetermined from the second cardiac electrical signal received by thesecond sensing channel 85. The second VT and VF interval counters may beupdated at block 415 based on RR's determined from the second cardiacelectrical signal received via the second sensing electrode vector 104.

In one instance, the tachyarrhythmia detector 92 may begin updatingsecond VT and VF interval counters at block 415 after the R-senseconfirmation threshold is reached at block 114. The process of updatingthe second VT and VF interval counters from an initialized zero countmay include confirming an R-wave at block 428 based on comparing anamplitude ratio to a ratio threshold as described in conjunction withblocks 122, 124, 126, and 128 of FIG. 7. If the R-wave sense isconfirmed at block 126, the R-wave confirmation rejection rule isupdated at block 428 based on the confirmed R-wave sensed event, andtachyarrhythmia detector 92 compares the event interval determined atblock 418 to VT and VF interval zones at block 415. The event intervaldetermined at block 418 is the time interval from the most recentlyconfirmed R-wave event time to the event time of the maximum absoluteamplitude of the time segment stored for the most recent R-wave sensedevent signal.

If the most recent R-wave sensed event signal is confirmed at block 428,the event interval may be compared at block 415 to VT and VF intervalzones defined to be the same as the interval zones applied at block 112to RR's determined from R-wave sensed event signals produced by thefirst sensing channel 83. If the event interval determined at block 418for a confirmed R-wave falls into the VT interval zone, the second VTinterval counter is increased at block 415. If the event interval fallsinto the VF interval zone, the second VF interval counter is increasedat block 415. In some examples, a combined VT/VF interval counter isincreased if the event interval falls into either a VT or VF intervalzone.

If one of the first VT or VF interval counters (or a combined VT/VFinterval counter) applied to RR's determined from the first sensingchannel 83 reaches an NID at block 132, tachyarrhythmia detector 92 maycompare the second VT and VF interval counters to second NIDrequirements at block 438. The second VT NID and the second VF NID usedby tachyarrhythmia detector 92 may be less than the VT NID and VF NIDapplied to the first VT and VF interval counters at block 132. Thesecond VT and VF interval counters begin to be updated after the R-senseconfirmation threshold is reached at block 114 in some examples. Assuch, the second VT and VF interval counters may have counts that areless than the first VT and VF interval counters (that are adjusted atblock 112). The counts of the second VT and VF interval counters mayfall behind the first VT and VF interval counts by the number ofintervals required to reach the R-sense confirmation threshold. Forexample, if a first VT interval counter is required to have a count ofat least 2 or the first VF interval counter is required to have a countof at least 3 in order for the R-sense confirmation threshold to bereached at block 114, the second VT or VF interval counter may have acount that is at least 2 or 3, respectively, less than the firstrespective VT or VF interval counter.

If a second NID is reached by one of the second VT or VF intervalcounters, “yes” branch of block 438, tachyarrhythmia detector 92determines if rejection criteria are met at block 440 based on thestatus of the rejection rules updated at block 428 through 436 asdescribed above in conjunction with FIG. 10. If the second NID is notreached at block 438 by neither of the second VT or VF intervalcounters, “no” branch of block 438, tachyarrhythmia detector 92 does notadvance to checking the rejection criteria at block 440. Rather,tachyarrhythmia detector 92 may wait for the first VT or VF NID to bereached at block 132 by the respective first VT or VF interval counterand for the respective second NID to be reached at block 438 by therespective second VT or VF interval counter. For instance, in order toadvance to block 440 to determine if rejection criteria are satisfiedand subsequently either detect VT at block 136 or withhold a VTdetection at block 140, the first VT interval counter is required toreach the first VT NID at block 132 and the second VT interval counteris required to reach the second VT NID at block 438. Similarly, in orderto advance to block 440 to determine if rejection criteria are satisfiedand subsequently either detect VF at block 136 or withhold a VFdetection at block 140, the first VF interval counter is required toreach the first VF NID at block 132 and the second VF interval counteris required to reach the second VF NID at block 438.

FIG. 12 is a diagram of circuitry included in the sensing circuit ofFIG. 4 according to another example. In FIG. 12, identically-numberedcomponents of sensing channels 83 and 85 correspond to like-numberedcomponents described in conjunction with and shown in FIG. 5. In theexample of FIG. 5, first sensing channel 83 is configured to senseR-waves by R-wave detector 66 in real time and produce R-wave sensedevent signals 68 that are passed to timing circuit 90 as the R-waves aresensed. Second sensing channel 85 is configured to pass the filtered,digitized output signal 78 to memory 82 for storage of second cardiacelectrical signal segments as triggered by R-wave sensed event signals68 from first sensing channel 83 without performing real-time R-wavesensing from the second cardiac electrical signal.

In the example of FIG. 12, second sensing channel 85 is configured topass the digitized filtered output signal 78 to memory 82 for storage ofsecond cardiac electrical signal segments as described above. Sensingchannel 85 is additionally configured to perform real-time R-wavesensing from the second cardiac electrical signal. In this case, secondsensing channel 85 includes rectifier 75 for rectifying the digitizedand bandpass filtered signal output of filter 74. The rectified signalis passed from rectifier 75 to R-wave detector 77. R-wave detector mayinclude a sense amplifier, comparator or other R-wave detectioncircuitry configured to apply an auto-adjusting R-wave sensing thresholdto the rectified signal for sensing an R-wave in response to apositive-going R-wave sensing threshold crossing.

Second sensing channel 85 may produce R-wave sensed event signals 79that are passed to timing circuit 90 in real time for use in determiningRRIs based on the second cardiac electrical signal. RRIs may bedetermined as the time interval or sample point count betweenconsecutively received R-wave sensed event signals 79. Timing circuit 90may pass RRIs determined from R-wave sensed event signals 79 from secondsensing channel 85 to tachyarrhythmia detector 92 for use in updatingsecond VT and VF interval counters based on RRIs determined fromreal-time sensing of R-waves by the second sensing channel 85.

In the flow chart 500 of FIG. 11, second VT and VF interval counters areupdated at block 415 by tachyarrhythmia detector 92 based on R-wavesconfirmed at block 428 as a result of post-processing of the storedsecond cardiac electrical signal segments. R-waves are not sensed by thesecond sensing channel 85 in real time in the example of FIG. 11 (thesignal segments may be recorded but R-wave sense event signals are notproduced in real time as R-waves are sensed based on an R-wave sensingthreshold). In FIG. 12, the second sensing channel 85 is configured tosense R-waves in real time from the second cardiac electrical signalreceived by the second sensing vector 104, and, as such, tachyarrhythmiadetector 92 may update second VT and VF interval counters based onreal-time sensing of R-waves by the second sensing channel 85.

FIG. 13 is a flow chart 550 of a method for detecting tachyarrhythmia byICD 14 according to another example in which the second sensing channel85 is configured for real-time sensing of R-waves from the secondcardiac electrical signal in addition to the control circuit 80 beingconfigured to confirm R-waves sensed by the first sensing channel 83 bypost-processing of the second cardiac electrical signal.Identically-numbered blocks in FIG. 13 correspond to like-numberedblocks shown in FIGS. 7 and/or 11 and described in conjunctiontherewith.

In the example of FIG. 13, at block 105, R-wave detector 77 of sensingchannel 85 produces R-wave sensed event signals 79 (shown in FIG. 12),e.g., in response to crossings of a second R-wave sensing threshold bythe second cardiac electrical signal. The second R-wave sensingthreshold may be an auto-adjusting threshold and may be different thanthe R-wave sensing threshold used by R-wave detector 66 of the firstsensing channel 83. Timing circuit 90 determines RRIs betweenconsecutive R-wave sensed event signals 79 received from second sensingchannel 85 at block 105 and passes the determined RRIs totachyarrhythmia detector 92. At block 419, tachyarrhythmia detector 92adjusts the second VT interval counter or the second VF intervalcounter, which may both be X of Y type counters, in response to each RRIdetermined at block 105. In this example, the second VT and VF intervalcounters of tachyarrhythmia detector 92 may be updated in real time,similar to the first VT and VF interval counters used to count RRIsdetermined from the first cardiac electrical signal. The second VT andVF interval counters may be updated on a beat-by-beat basis withoutrequiring the R-sense confirmation threshold to be reached first (block114).

If the tachyarrhythmia detector 92 determines that a first VT NID orfirst VF NID is reached at block 132, the tachyarrhythmia detector 92compares the second VT and VF interval counters to a second VT NID andsecond VF NID, respectively, at block 439. In this case, the second VTNID and second VF NID may be the same as the first VT NID and the firstVF NID since all of the first and second VT interval counters and thefirst and second VF interval counters are being updated in response toR-waves that are sensed in real time. If the second VT or VF NID has notbeen reached (“no” branch of block 430), the tachyarrhythmia detector 92may return to block 132 to wait for the VT or VF NID thresholds to bereached based on R-waves sensed in real time by both the respectivefirst and second sensing channels 83 and 85.

If a second VT or VF NID is reached at block 439 when a correspondingfirst VT or VF NID is reached at block 132, the tachyarrhythmia detector92 determines if rejection criteria are satisfied at block 440 asdescribed previously in conjunction with FIG. 10. A pending VT of VFdetection based on tachyarrhythmia intervals being detected in real timefrom both the first and second cardiac electrical signals is eitherwithheld at block 140 in response to rejection criteria being met or theVT or VF detection is confirmed at block 135 if the rejection criteriaare not satisfied. Control circuit 80 controls the therapy deliverycircuit to deliver an anti-tachyarrhythmia therapy at block 138 inresponse to the VT or VF detection.

FIG. 14 is a flow chart 600 of a method performed by ICD 14 forwithholding a VT or VF detection in response to the gross morphologyrejection rule (block 430, FIGS. 10, 11 and 13) being satisfied. Blocks102 through 114, 132, 136, 138, and 140 of FIG. 14 correspond toidentically-numbered blocks described above in conjunction with FIGS.10, 11 and 13.

As described above, if the R-sense confirmation threshold is reached atblock 114, the control circuit 80 applies a notch filter to the stored,second cardiac electrical signal segment at block 416. The notch filterapplied at block 416 may correspond to the filter described inconjunction with FIG. 6. The notch-filtered, second cardiac electricalsignal segments are used at blocks 602, 604 and 606 to determinemorphology parameters relating to baseline noise (block 602), maximumsignal amplitude (block 604) and maximum signal width (block 606).Examples of methods that may be performed by control circuit 80 fordetermining a baseline noise parameter, determining a signal amplitudeparameter, and determining a signal width parameter are described belowin conjunction with FIGS. 15, 16 and 17, respectively. The methodsperformed at blocks 602, 604 and 608 may correspond to the determinationof gross morphology parameters at block 420 of FIGS. 10, 11 and 13.

These parameters are compared to noise detection criteria at block 608.If the noise detection criteria are satisfied at block 608, the secondcardiac signal segment being analyzed is determined to be a noisysegment at block 610. The noise contamination of the signal segment maybe caused by muscle (myoelectric) noise, EMI or other non-cardiacelectrical noise sources.

In response to detecting a noisy segment at block 610 (“yes” branch),control circuit 80 increases a noisy segment count at block 614 to trackthe number of segments identified as being noisy based on the morphologyparameters after the R-wave confirmation threshold is reached at block114. The status of the gross morphology rejection rule is updated atblock 430 based on the value of the noisy segment count. When thesegment is not detected as being noisy, “no” branch of block 610, thegross morphology rejection rule may be updated at block 430 to track thenumber of noisy segments that are detected out of a predetermined numberof segments analyzed or out of the most recent R-wave sensed eventsignals. The gross morphology rejection rule may be satisfied based onthe noisy segment count value. If a threshold number of noisy segmentsare detected, the rejection rule is satisfied at block 430. For example,if at least two out of the most recent eight (or other predeterminednumber) analyzed cardiac electrical signal segments are classified asnoisy, the rejection rule is updated as being satisfied at block 430. Ifless than two out of the most recent eight analyzed cardiac electricalsignal segments are classified as noisy, the rejection rules is updatedas being unsatisfied.

The next signal segment is fetched at block 612 and undergoes the sameanalysis at blocks 602 through 610 to determine whether the segment isnoisy or not based on the noise detection criteria applied at block 608.As described below in conjunction with FIG. 19, the next signal segmentto undergo analysis for detecting a noisy signal segment may notcorrespond to the next consecutive R-wave sensed event signal. In somecases, the next signal segment that is fetched at block 612 correspondsto an R-wave sensed event signal that occurs at least 300 ms or anotherpredetermined time limit after the R-wave sensed event signal thattriggered storage of the signal segment currently being analyzed. Inthis way, signal segments analyzed for being identified as noisy signalsegments may sometimes be consecutive and at other times benon-consecutive. R-wave sensed event signals that occur at an RRI lessthan the predetermined time limit may be skipped for the purposes ofdetermining gross morphology parameters.

In response to the NID being reached at block 132, based on RRIsdetermined from the cardiac electrical signal received via the firstsensing vector 102 by the first sensing channel 83, control circuit 80determines if rejection criteria are satisfied at block 440. In theexample of FIG. 14, control circuit 80 checks the status of the grossmorphology rejection rule updated at block 430. If the gross morphologyrejection rule is satisfied when the NID is reached at block 132, e.g.,if at least two out of the most recent eight signal segments analyzed,as given in the example above, are detected as noisy segments, a VT orVF detection based on the NID being reached at block 132 is withheld atblock 140; no therapy is delivered.

This process of analyzing signal segments for detecting noisy segmentsmay be repeated as long as the R-sense confirmation threshold is stillbeing met or until a VT or VF is detected. VT or VF is detected at block136 when the NID is reached and the gross morphology rejection rule (andany other rejection rules applied at block 440 as described previouslyherein) is not satisfied (“no” branch of block 440). Control circuit 80controls therapy delivery circuit 84 to deliver a therapy, e.g., ATP ora shock therapy, according to programmed therapies. It is to beunderstood that in some examples, the VT or VF detection made at block136 may require a second NID to be reached based on RRIs determined fromthe second cardiac electrical signal as described in conjunction withFIGS. 11 and 13.

FIG. 15 is a flow chart 700 of a method performed by ICD 14 fordetermining a morphology parameter correlated to baseline noise at block602 of FIG. 14 according to one example. The morphology parametercorrelated to baseline noise is referred to herein as a baseline noiseparameter and in the example of FIG. 15 the morphology parametercorrelated to baseline noise is a count of signal pulses during thesignal segment.

At block 702 a first order differential signal is determined from thesecond cardiac electrical signal segment. In one example, the storedsegment is at least 360 ms long so that a first order differentialsignal is determined using sample points over a 360 ms segment of thesecond cardiac electrical signal. The 360 ms segment may include samplepoints preceding and following an R-wave sensed event signal. Forinstance, the 360 ms segment may include 92 sample points when thesampling rate is 256 Hz with 24 of the sample points occurring after theR-wave sensed event signal that triggered the storage of the signalsegment and 68 sample points extending from the R-wave sensed eventsignal earlier in time from the R-wave sensed event signal. Byprocessing relatively fewer sample points occurring after the R-wavesensed event signal, control circuit 80 does not need to wait long afterthe R-wave sensed event signal for acquiring all sample points needed todetermine the baseline noise parameter (and other gross morphologyparameters), thereby minimizing the signal processing delay in detectingnoise and updating the gross morphology rejection rule. The first orderdifferential signal is computed by computing successive differences,A(n)−A(n−1), between sample points, where n is the sample point number,ranging from 1 to 92 in the example given above, A(n) is the amplitudeof the nth sample point and A(n−1) is the amplitude of the immediatelypreceding n−1 sample point.

At block 704, zero crossings are set by identifying consecutive samplepoints of the differential signal having opposite polarity. For examplea positive sample point followed by a negative sample point isidentified as a zero crossing, or a negative sample point followed by apositive sample point is identified as a zero crossing. Control circuit80 compares the absolute values of the two signal sample pointsidentified as a zero crossing. The sample point of the differentialsignal having the smaller absolute value amplitude is set to zeroamplitude to clearly demark each zero crossing with two consecutive zerocrossings defining a baseline noise pulse.

The differential signal with zero crossings set is rectified at block706. At block 708, a noise pulse amplitude threshold is determined. Thenoise pulse amplitude threshold may be determined based on the maximumamplitude of the rectified differential signal over the entire segmentbeing analyzed, over 360 ms in the example given above. The noise pulseamplitude threshold may be set to a portion or percentage of the maximumamplitude of the rectified differential signal. For instance, the noisepulse amplitude threshold may be set to be one-eighth of the maximumamplitude of the rectified differential signal.

At block 710, signal pulses within the segment being analyzed areidentified and counted. Signal pulses are identified by comparing thesample point amplitudes of the rectified differential signal occurringbetween two consecutive zero crossings to the noise pulse amplitudethreshold. If the rectified differential signal amplitude crosses thenoise pulse amplitude threshold, a signal pulse is identified andcounted at block 710. The control circuit 80 may advance through theentire stored signal segment, or selected portion thereof that extendsearlier in time than the QRS complex to include a baseline signalportion, to identify and count all pulses exceeding the noise pulseamplitude threshold in the segment under analysis. The baseline noiseparameter determined at block 602 of FIG. 14 is the count of identifiedsignal pulses determined at block 710. Other techniques that may beimplemented for determining a baseline noise parameter at block 602,such as a muscle noise pulse count, are generally disclosed in U.S. Pat.No. 8,435,185 (Ghanem, et al.), incorporated herein by reference in itsentirety.

FIG. 16 is a flow chart 800 of a method for determining an amplitudeparameter at block 605 of FIG. 14 according to one example. At block802, the notch-filtered signal segment is rectified, e.g., a 360 mssegment as described above in conjunction with FIG. 15 is rectified.Control circuit 80 determines the maximum absolute amplitude of therectified, notch-filtered segment at block 804. The amplitudes of allsample points of the rectified signal segment under analysis are summedat block 806. At block 808, the normalized rectified amplitude (NRA) isdetermined based on the maximum absolute amplitude determined at block804 and the summed sample point amplitudes determined at block 806. Inone example, the NRA is determined as the ratio of a predeterminedmultiple of the summation of all sample point amplitudes of thenotch-filtered and rectified signal segment to the maximum absoluteamplitude. For instance, the NRA may be determined as four times thesummed amplitudes divided by the maximum absolute amplitude, which maybe truncated to an integer value. This NRA may be determined as theamplitude parameter at block 604 of FIG. 14. The higher this amplitudeparameter value is, the more likely that the signal segment contains avalid R-wave of a VT or VF episode. The signal segment may be identifiedas a “shockable beat” when the amplitude parameter is greater than ashockable beat amplitude threshold as described below in conjunctionwith FIG. 18.

FIG. 17 is a flow chart 810 of a method that may be performed by controlcircuit 80 for determining a signal width parameter at block 606 of FIG.14. Blocks 802 and 804 correspond to identically-numbered blocksdescribed above in conjunction with FIG. 16. The notch-filtered,rectified signal segment determined at block 802 and the maximumabsolute amplitude of the rectified signal segment determined at block804 are used for determining the signal amplitude parameter at block 604of FIG. 14 and the signal width parameter at block 606 of FIG. 14. Inother examples, however, the differential signal with set zero crossingsused to determine the pulse count as described in conjunction with FIG.15 may also be used to determine the signal amplitude and signal widthparameters instead of the notch-filtered, rectified signal segment.

Control circuit 80 determines a pulse amplitude threshold at block 812based on the maximum absolute amplitude. This pulse amplitude thresholdis used for identifying a signal pulse having the maximum signal widthin the second cardiac electrical signal segment. The pulse amplitudethreshold determined at block 812 may be a different threshold than thenoise pulse amplitude threshold determined at block 708 of FIG. 15. Forexample, the pulse amplitude threshold used for determining the signalwidth parameter may be set to half the maximum absolute amplitude of therectified, notch-filtered signal segment whereas the noise pulseamplitude threshold used in FIG. 15 to determine a count of signalpulses may be set to one-eighth the maximum amplitude of rectified,differential signal segment.

The signal amplitude parameter determined by the method of FIG. 16 andthe signal width parameter determined by the method of FIG. 17 are usedto identify signal pulses that are more likely to be an R-wave of apotential VT or VF rhythm than a baseline noise pulse. A signal segmentthat has a relatively large signal amplitude parameter and/or relativelylarge signal width parameter is potentially a heartbeat of a VT or VFrhythm rather than noise, and is referred to herein as a “potentialshockable beat.” When a potential shockable beat is detected based on alarge signal amplitude and/or signal width parameter, more stringentcriteria may be applied for detecting the signal segment as a noisysegment to avoid withholding a VT or VF detection based on a potentialshockable beat. The signal segment including a potential shockable beatis required to meet higher noise detection criteria in order to justifycounting the signal segment as a noisy segment at block 614 (FIG. 14),which may lead toward satisfying the gross morphology rejection rule andwithholding a VT or VF detection. Otherwise a potential shockable beatthat is counted as a noisy segment based on less stringent noisedetection criteria may lead to withholding or delay of a VT or VFdetection during a true tachyarrhythmia episode.

At block 814, control circuit 80 determines the signal width for allpulses of the segment as the number of sample points (or correspondingtime interval) between a pair of consecutive zero crossings of therectified signal. The maximum peak of each pulse is determined at block816. All pulses having a maximum peak that is greater than or equal tothe pulse amplitude threshold determined at block 812, e.g., greaterthan half the maximum absolute amplitude, are identified. Of the pulseshaving a maximum peak amplitude that is at least the pulse amplitudethreshold, the pulse having the greatest pulse width is identified atblock 820. This maximum pulse width is determined as the signal widthparameter at block 606 of FIG. 14.

FIG. 18 is a flow chart 900 of a method for comparing the grossmorphology parameters to noise detection criteria and detecting thesegment as a noisy segment. The process of flow chart 900 may beperformed at blocks 608 and 610 of FIG. 14. At block 902, controlcircuit 80 fetches the gross morphology parameters determined at blocks602, 604 and 606 for comparison to noise detection criteria. The NRA iscompared to a potential shockable beat amplitude threshold at block 904.When the NRA is determined as described above in conjunction with FIG.16, the potential shockable beat amplitude threshold may be set between100 and 150, and to 140 in some examples. If the NRA is greater than thethreshold, the signal segment may likely include a true R-wave of a VTor VF episode and is therefore a potential shockable beat (if it isoccurring at a VT or VF interval).

At block 906, the maximum pulse width is compared to a potentialshockable beat width threshold. If the NRA and the maximum pulse widthfor the signal segment are both greater than the respective amplitudeand width thresholds at blocks 904 and 906, the segment is identified asa potential shockable beat at block 908. In one example, the potentialshockable beat width threshold is set to 20 sample points when thesampling rate is 256 Hz. The signal pulse count determined as thebaseline noise parameter (as described in conjunction with FIG. 15) iscompared to a first noisy segment threshold at block 910. If the signalpulse count is greater than the noisy segment threshold, e.g., greaterthan 12, the segment is detected as a noisy segment at block 916. Thenoisy segment count is increased by one at block 614 of FIG. 14 inresponse to detecting the noisy segment.

If the pulse count is less than the first noisy segment threshold, “no”branch of block 910, the segment is not detected as a noisy segment, andthe process returns to block 902 to fetch the gross morphologyparameters for the next signal segment, as long as the R-senseconfirmation threshold is still being reached at block 114 and VT or VFhas not been detected.

If the segment is not identified as a potential shockable beat, “no”branch of block 904 or “no” branch of block 906, the count of identifiedsignal pulses is compared to a second noisy segment threshold at block914. The second noisy segment threshold may be lower than the firstnoisy segment threshold. If the segment is not identified as a potentialshockable beat based on the amplitude parameter and the signal widthparameter, less stringent criteria, e.g., a lower noisy segmentthreshold, may be applied for classifying the segment as a noisysegment. In one example, the second noisy segment threshold is six. Ifthe pulse count exceeds the second noisy segment threshold, the segmentis detected as a noisy segment at block 916 and will be counted by thecontrol circuit 80 at block 614 of FIG. 14. Otherwise, the segment isnot detected as a noisy segment (“no” branch of block 914), and thegross morphology parameters for the next segment are fetched at block902.

The gross morphology rejection rule is satisfied when the value of thenoisy segment count reaches or exceeds a rejection threshold, e.g., twonoisy segments counted out of the eight most recent analyzed segments asdescribed above in conjunction with FIG. 14. The segment is classifiedas noisy based on a count of signal pulses, positive- or negative-goingin the originally stored segment, that have a differential signalabsolute amplitude greater than a noise pulse amplitude threshold. Thenumber of signal pulses required to detect a noisy segment is smallerwhen the segment is identified as a potential shockable beat based on arelatively large amplitude parameter and/or a relatively large signalwidth parameter than the number of signal pulses required to detect anoisy segment when the segment is not identified as a potentialshockable beat. In the example of FIG. 18, both the amplitude parameterand the signal width parameter are required to exceed a respectiveamplitude threshold and width threshold in order to identify the segmentas a potential shockable beat. In other examples, only one of theamplitude parameter or the signal width parameter may be required toexceed its respective threshold in order to identify the segment as apotential shockable beat.

FIG. 19 is a timing diagram 950 depicting time segments over which themorphology parameter correlated to baseline noise, amplitude parameterand signal width parameter may be determined. An R-wave sensed eventsignal 952 occurring at time 962 may cause the R-wave confirmationthreshold to be reached. The next R-wave sensed event signal 953 at time963 triggers storage of a time segment 970 of the second cardiacelectrical signal segment. A portion 972 of the time segment 970 extendsearlier than the R-wave sensed event signal 953 and a portion 974extends after the R-wave sensed event signal 953. The gross morphologyparameters may be determined for the time segment 970 and analyzed fordetermining if noise detection criteria are satisfied for detectingsegment 970 as a noisy segment according to the methods of FIGS. 15through 18.

The next R-wave sensed event signal 954 occurs within a predeterminedtime limit 980 from time 963. As such, analysis of a time segmentcorresponding to R-wave sensed event signal 954 is skipped. A secondcardiac electrical signal segment may be stored for R-wave sensed eventsignal 954 and used in other signal analysis for use in confirming asensed R-wave and/or determining the status of other rejection rules,but is not used in the gross morphology rejection rule analysis in someexamples. Time limit 980 may be 300 ms in one example but may be longeror shorter than this value. The time limit 980 may be set to enableprocessing time of the analysis of a time segment 970 to be performedprior to starting analysis of the next time segment. The next R-wavesensed event signal 955 occurs at time 965, after time limit 980expires. A time segment 975 buffered in memory 82 in response to R-wavesensed event signal 955 is analyzed for determining if it is a noisysegment.

R-wave sensed event 956 at time 966 occurs after time limit 980 expires,so time segment 976 of the second cardiac electrical signal is analyzedaccording to the methods of FIGS. 15-18. R-wave sensed event signal 957occurs within the time limit 980 after R-wave sensed event signal 956 soanalysis of a corresponding time segment is skipped. The next timesegment 978 corresponding to R-wave sensed event 958 at time 968 is theearliest occurring R-wave sensed event after time limit 980 expires andis therefore analyzed for determining if the segment 978 is a noisysegment. As such, time segments of the second cardiac electrical signalmay sometimes correspond to consecutive R-wave sensed event signals andsometimes correspond to non-consecutive R-wave sensed event signals whenone or more time segments corresponding to R-wave sensed events thatoccur within the predetermined time limit 980 are skipped.

Thus, a method and apparatus for withholding a ventriculartachyarrhythmia detection in response to detecting noise contaminationof cardiac electrical signal segments in an extra-cardiovascular ICDsystem have been presented in the foregoing description with referenceto specific embodiments. In other examples, various methods describedherein may include steps performed in a different order or combinationthan the illustrative examples shown and described herein. It isappreciated that various modifications to the referenced embodiments maybe made without departing from the scope of the disclosure and thefollowing claims.

1. A medical device system comprising: a sensing circuit comprising: afirst sensing channel configured to receive a first cardiac electricalsignal via a first sensing electrode vector coupled to the medicaldevice system and to sense R-waves in response to crossings of a sensingamplitude threshold by the first cardiac electrical signal, and a secondsensing channel configured to receive a second cardiac electrical signalvia a second sensing electrode vector coupled to the medical devicesystem and different than the first sensing electrode vector; a memory;and a control circuit coupled to the sensing circuit and the memory, thecontrol circuit configured to: store a time segment of the secondcardiac electrical signal in the memory for each of the plurality ofR-waves sensed by the first sensing channel; for each of a plurality ofthe stored time segments, determine a morphology parameter correlated tosignal noise from the stored time segment; and detect the stored timesegment as being a noisy signal segment based on the morphologyparameter determined for the respective stored time segment; andwithhold detection of a tachyarrhythmia episode in response to detectingat least a threshold number of the stored time segments as noisy signalsegments.
 2. The system of claim 1, further comprising a notch filter,wherein the time segments of the second cardiac electrical signal isfiltered by the notch filter prior to determining the morphologyparameter.
 3. The system of claim 1, wherein the control circuit isfurther configured to: determine a plurality of intervals betweensuccessive ones of the R-waves sensed by the first sensing channel; anddetermine the morphology parameter from the stored time segment inresponse to at least a predetermined number of the plurality ofintervals being less than a tachyarrhythmia detection interval.
 4. Thesystem of claim 1, wherein the control circuit is configured todetermine the morphology parameter by: identifying signal pulsesoccurring during the time segment of the second cardiac electricalsignal; and determining a count of the identified signal pulses.
 5. Thesystem of claim 4, wherein the control circuit is configured to identifythe signal pulses by: identifying zero crossings of the second cardiacelectrical signal during the time segment; and identifying a signalpulse in response to the second cardiac electrical signal crossing anoise pulse amplitude threshold between two consecutive zero crossings.6. The system of claim 1, wherein the control circuit is furtherconfigured to: determine from the stored time segment at least one of asignal width parameter or a signal amplitude parameter; detect apotential shockable beat based on at least one of the signal widthparameter or the signal amplitude parameter; compare the morphologyparameter to a first noisy segment threshold in response to detectingthe potential shockable beat; determine that the stored time segment isa noisy signal segment in response to the morphology parameterdetermined for the respective stored time segment being greater than orequal to the first noisy segment threshold.
 7. The system of claim 6,wherein the control circuit is further configured to: compare themorphology parameter to a second noisy segment threshold in response tonot detecting a potential shockable beat based on the at least one ofthe signal width parameter or the signal amplitude parameter; anddetermine that the stored time segment is a noisy signal segment inresponse to the morphology parameter determined for the respectivestored time segment being greater than or equal to the second noisysegment threshold, the second noisy segment threshold less than thefirst noisy segment threshold.
 8. The system of claim 6, wherein thecontrol circuit is configured to determine the signal width parameterby: identifying zero crossings of the second cardiac electrical signalduring the stored time segment; identifying a signal pulse in responseto the second cardiac electrical signal crossing a pulse amplitudethreshold between two consecutive zero crossings; determining a pulsewidth of each identified signal pulse as a time interval between the twoconsecutive zero crossings of each respective identified signal pulse;determining the signal width parameter as a maximum one of thedetermined pulse widths.
 9. The system of claim 6, wherein the controlcircuit is further configured to: determine the signal amplitudeparameter by: determining a summation of absolute amplitudes of allsample points of the stored time segment; and normalizing the summationby a maximum sample point amplitude of the time segment; compare theamplitude parameter to a shockable beat amplitude threshold; compare thesignal width parameter to a shockable beat width threshold; and detectthe potential shockable beat when the amplitude parameter is greaterthan the shockable beat amplitude threshold and the signal widthparameter is greater than the shockable beat width threshold.
 10. Thesystem of claim 1, wherein the control circuit is further configured to:increase a count of noisy signal segments in response to detecting thenoisy signal segment; determine a plurality of intervals betweensuccessive ones of the R-waves sensed by the first sensing channel;compare a value of the count to a rejection threshold in response todetecting a predetermined number of the plurality of intervals beingless than a tachyarrhythmia detection interval, and withhold thedetection of the tachyarrhythmia episode in response to the value of thecount of noisy signal segments being equal to or greater than therejection threshold.
 11. The system of claim 1, wherein the controlcircuit is configured to: determine the morphology parameter for a firsttime segment of the second cardiac electrical signal corresponding to afirst sensed R-wave; determine if a next sensed R-wave occurs within apre-determined time limit from the first sensed R-wave; and skipdetermining the morphology parameter from a time segment of the secondcardiac electrical signal corresponding to the next sensed R-wave inresponse to the next sensed R-wave occurring within the pre-determinedtime limit.
 12. The system of claim 1, further comprising anextra-cardiovascular lead coupled to the ICD, wherein at least oneelectrode of the first sensing electrode vector is carried by theextra-cardiovascular lead.
 13. The system of claim 1, wherein the firstsensing electrode vector has a first inter-electrode spacing and thesecond electrode vector has a second inter-electrode spacing, the secondinter-electrode spacing being greater than the first inter-electrodespacing.
 14. The system of claim 1, further comprising a therapydelivery circuit configured to deliver an anti-tachyarrhythmia therapyto a patient's heart via electrodes coupled to the medical devicesystem, wherein the control circuit is further configured to: increase acount of noisy segments in response to detecting the noisy signalsegment; determine a plurality of intervals between successive ones ofthe of R-waves sensed by the first sensing channel; compare a value ofthe count of noisy segments to a rejection threshold in response todetecting a predetermined number of the plurality of intervals beingless than a tachyarrhythmia detection interval; detect thetachyarrhythmia episode in response to the value of the count being lessthan the rejection threshold; and control the therapy delivery circuitto deliver the anti-tachyarrhythmia therapy in response to detecting thetachyarrhythmia episode.
 15. A method comprising: sensing a plurality ofR-waves by a first sensing channel of a sensing circuit of a medicaldevice system in response to crossings of a sensing amplitude thresholdby a first cardiac electrical signal, the first cardiac electricalsignal received by the first sensing channel via a firstextra-cardiovascular sensing electrode vector coupled to the medicaldevice system; storing a time segment of a second cardiac electricalsignal in a memory of the for each of the plurality of R-waves sensed bythe first sensing channel, the second cardiac electrical signal receivedvia a second sensing electrode vector coupled to the medical devicesystem and different than the first sensing electrode vector; for eachof a plurality of the stored time segments, determining a morphologyparameter correlated to signal noise from the stored time segment;detecting the stored time segment as being a noisy signal segment basedon the morphology parameter determined for the respective stored timesegment; and withholding detection of a tachyarrhythmia episode inresponse to detecting at least a threshold number of the stored timesegments as noisy signal segments.
 16. The method of claim 15, furthercomprising filtering the time segment of the second cardiac electricalsignal by a notch filter prior to determining the morphology parameter.17. The system of claim 15, further comprising: determining a pluralityof intervals between successive ones of the R-waves sensed by the firstsensing channel; determining that at least a predetermined number of theplurality of intervals are less than a tachyarrhythmia detectioninterval; and determining the morphology parameter in response to thepredetermined number of the plurality of intervals being less than thetachyarrhythmia detection interval.
 18. The method of claim 15, whereindetermining the morphology parameter comprises: identifying signalpulses occurring during the time segment of the second cardiacelectrical signal; and determining a count of the identified signalpulses.
 19. The method of claim 18, wherein identifying the signalpulses comprises: identifying zero crossings of the second cardiacelectrical signal during the time segment; and determining the secondcardiac electrical signal crosses a noise pulse amplitude thresholdbetween two consecutive zero crossings; and identifying a signal pulsein response to the second cardiac electrical signal crossing the noisepulse amplitude threshold between two consecutive zero crossings. 20.The method of claim 15, further comprising: determining at least one ofa signal width parameter or a signal amplitude parameter for the storedtime segment; detecting a potential shockable beat based on at least oneof the signal width parameter or the signal amplitude parameter;comparing the morphology parameter to a first noisy segment threshold inresponse to detecting the potential shockable beat; determining that themorphology parameter determined for the respective stored signal segmentis greater than or equal to the first noisy segment threshold; anddetecting the stored time segment as a noisy signal segment in responseto the morphology parameter determined for the respective stored signalsegment being greater than or equal to the first noisy segmentthreshold.
 21. The method of claim 20, further comprising: comparing themorphology parameter to a second noisy segment threshold in response tonot detecting a potential shockable beat based on the at least one ofthe signal width parameter or the signal amplitude parameter;determining that the morphology parameter is greater than or equal tothe second noisy segment threshold, the second noisy segment thresholdless than the first noisy segment threshold; and detecting the storedtime segment as a noisy signal segment in response to the morphologyparameter being greater than or equal to the second noisy segmentthreshold, the second noisy segment threshold less than the first noisysegment threshold.
 22. The method of claim 20, wherein determining thesignal width parameter comprises: identifying zero crossings of thesecond cardiac electrical signal during the stored time segment;determining that the second cardiac electrical signal crosses a pulseamplitude threshold between two consecutive zero crossings; identifyinga signal pulse in response to the second cardiac electrical signalcrossing the pulse amplitude threshold between two consecutive zerocrossings; determining a pulse width of each identified signal pulse asa time interval between the two consecutive zero crossings of eachrespective identified signal pulse; determining the signal widthparameter as a maximum one of the determined pulse widths.
 23. Themethod of claim 20, further comprising: determining the amplitudeparameter by: determining a summation of absolute amplitudes of allsample points of the time segment; and normalizing the summation by amaximum sample point amplitude of the time segment; comparing theamplitude parameter to a shockable beat amplitude threshold; determiningthat the amplitude parameter is greater than the shockable beatamplitude threshold; comparing the signal width parameter to a shockablebeat width threshold; determining that the signal width parameter isgreater than the shockable beat width threshold; and detecting thepotential shockable beat when the amplitude parameter is greater thanthe shockable beat amplitude threshold and the signal width parameter isgreater than the shockable beat width threshold.
 24. The method of claim15, further comprising: increasing a count of noisy signal segments inresponse to detecting the noisy signal segment; determining a pluralityof intervals between successive ones of the R-waves sensed by the firstsensing channel; determining that a predetermined number of theplurality of intervals are less than a tachyarrhythmia detectioninterval; comparing a value of the count to a rejection threshold inresponse to detecting the predetermined number of the plurality ofintervals being less than the tachyarrhythmia detection interval, anddetermining that the value of the count of noisy signal segments isequal to or greater than the rejection threshold; and withholding thedetection of the tachyarrhythmia episode in response to the value of thecount of noisy signal segments being equal to or greater than therejection threshold.
 25. The method of claim 15, further comprising:determining the morphology parameter for a first time segment of thesecond cardiac electrical signal corresponding to a first sensed R-wave;determining if a next sensed R-wave occurs within a pre-determined timelimit from the first sensed R-wave; determining that the next sensedR-wave occurs within the pre-determined time limit; and skippingdetermining the morphology parameter from a time segment of the secondcardiac electrical signal corresponding to the next sensed R-wave inresponse to the next sensed R-wave occurring within the pre-determinedtime limit.
 26. The method of claim 15, further comprising sensing thefirst cardiac electrical signal via at least one electrode carried by anextra-cardiovascular lead coupled to the ICD.
 27. The method of claim15, further comprising: sensing the first cardiac electrical signal bythe first sensing electrode vector having a first inter-electrodespacing; and sensing the second cardiac electrical signal by the secondsensing electrode vector having a second inter-electrode spacing, thesecond inter-electrode spacing being greater than the firstinter-electrode spacing.
 28. The method of claim 1, further comprising:increasing a count of noisy segments in response to detecting the noisysignal segment; determining a plurality of intervals between successiveones of the of R-waves sensed by the first sensing channel; detecting apredetermined number of the plurality of intervals being less than atachyarrhythmia detection interval; comparing a value of the count ofnoisy segments to a rejection threshold in response to detecting thepredetermined number of the plurality of intervals being less than thetachyarrhythmia detection interval; determining the value of the countis less than the rejection threshold; detecting the tachyarrhythmiaepisode in response to the value of the count being less than therejection threshold; and controlling a therapy delivery circuit of themedical device system to deliver the anti-tachyarrhythmia therapy inresponse to detecting the tachyarrhythmia episode.
 29. A non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of an extra-cardiovascularimplantable cardioverter defibrillator (ICD), cause theextra-cardiovascular ICD to: sense a plurality of R-waves by a firstsensing channel of a sensing circuit of the extra-cardiovascular ICD inresponse to crossings of a sensing amplitude threshold by a firstcardiac electrical signal, the first cardiac electrical signal receivedby the first sensing channel via a first extra-cardiovascular sensingelectrode vector coupled to the extra-cardiovascular ICD; store a timesegment of a second cardiac electrical signal in ICD memory in responseto each one of the plurality of R-waves sensed by the first sensingchannel, the second cardiac electrical signal received via a secondextra-cardiovascular sensing electrode vector by a second sensingchannel of the extra-cardiovascular ICD; for each one of a plurality ofthe stored time segments determine a morphology parameter correlated tosignal noise from the stored time segment; and detect the stored timesegment as being a noisy signal segment based on the morphologyparameter determined for the respective stored time segment; andwithhold detection of a tachyarrhythmia episode in response to detectingat least a threshold number of the stored time segments as noisy signalsegments.